Reimplement selection in the terminal

[Tyriar]

This change re-implements the selection model within the terminal, taking the responsibility away from the browser and into xterm.js. This may seem like a lot of work to replace something we already get for free, but selection in the browser was never meant to work with the unique rendering model of xterm.js and other quirks with terminals. As such, this implementation will fix a bunch of bugs, probably most notably is the ability to select and copy more than a single viewport worth of data.

Checklist

• Selection
  • Drag selection works within viewport
    • Works with wide chars
  • Drag selection on bounds of viewport scrolls the buffer
  • Double click
    • Selects a word
    • Selects whitespace
    • Works with wide chars
  • Triple click selects a line
    • Works with wide chars
    • Dragging selects multiple full lines
  • Double and triple clicks allow the selection to be expanded
  • Selection is disabled when mouse events are enabled
  • Ensure the selection is being cleared when appropriate (eg. if a line is removed in the middle of the selection)
  • Select all
    • Basic implementation
    • cmd+a works
  • Zoom
    • Selection is drawn in correct position (fixed after Set row height when char size is changed #680 merged)
  • Shift+click should expand selection when there is already a selection (send to terminal as normal if there is no selection)
• Copy
  • Selection can be copied to clipboard via cmd/ctrl+c
  • Selection can be copied to clipboard via context menu
  • Works with wide chars
  • Alt buffer
    • Copying in the alt buffer will grab from the alt buffer, not the normal buffer
• Paste via context menu
• API
  • There is sufficient API to reimplement copy outside of xterm.js (eg. for custom hotkeys in Electron)
• Testing
  • Ensure there is sufficient test coverage
  • Ensure selection works within tmux
  • Ensure copy works within tmux
  • Do some stress testing
  • Test several browsers
    • Chrome v58, Linux
    • Chrome, Mac
    • Safari v10, Mac
    • Chrome v59, Windows
    • Edge v40, Windows
    • IE v11, Windows (demo broken atm Demo doesn't work in IE11 #684)
• Polish/clean up
  • Resolve or accept all TODOs
  • Determine whether to keep or discard CircularList id (removed)
  • See if the cursor state can remain static when a selection is happening (as opposed to unfocused) (deferred to Improve focus/blur state #681)
  • Go through related issues and link them to this PR
  • Update PR write up (will defer write up of features to the release notes)

Linked issues

Below is a list of items that this change may or may not fix, originally composed on microsoft/vscode#9958. These items may or may not get resolved depending on how the implementation pans out, but checking them off here will make it easier to track what's changing:

• Double clicking on the prompt line being flaky due to focus shifting from .xterm to the text area
• Double clicking on a URL/path to highlight the whole path (ie. don't use / and similar chars as separators)
• Selecting and copying multiple pages of the buffer Selection is only possible on the text in the current viewport #69
• Selection goes crazy when you start dragging and then use mouse wheel Selection goes crazy when you start dragging and then use mouse wheel #68
• Making the selection's line height exact so there are never any gaps between lines Terminal rendering shows space between lines with zoomlevel:1 microsoft/vscode#18588
• Don't drop the selection when there's a content change or viewport scroll text-selection is lost when data is written outside of the viewport #468 Spinners in console applications make selection difficult #318
• Allow selection to be retained in terminal when not focused (like editor) 'Terminal: Copy Selection' no way to be executed from the command palette microsoft/vscode#9761
• Add select all command Select all does not work #307 "Select all" not available in terminal microsoft/vscode#15199
• Selection is dropped sometimes when right clicking outside of selection Right clicking on a portion of the terminal that is not selected sometimes drops selection microsoft/vscode#23866
• Black marker is shown when context menu is closed A black marker is shown when context menu is closed microsoft/vscode#9758
• 'Terminal: Copy Selection' no way to be executed from the command palette 'Terminal: Copy Selection' no way to be executed from the command palette microsoft/vscode#9761 (this can be fixed downstream)
• Cannot select link in terminal Cannot select link in terminal microsoft/vscode#25155
• Copying to clipboard always trims the selection Copying to clipboard always trims the selection #673
• Selection should not be visible during mouse interaction mode Selection should not be visible during mouse interaction mode #140
• Improve tmux support Improve tmux support #456
• Tmux ignores mouse scroll events/shits itself on selection events Tmux ignores mouse scroll events/shits itself on selection events #329
• Copy/paste does not work in Edge Copy/paste does not work in Edge #401

To help reduce the diff I pulled out some of the changes into smaller PRs: #669, #668, #667

Known issues

• Select all via context menu doesn't work in Edge/Chrome

Follow ups

• Consider adding smarter double click to select word logic #682 Consider adding smarter double click to select word logic
• Improve focus/blur state #681 Improve focus/blur state

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Fixes #207
Related microsoft/vscode#9958

[coveralls]

Coverage increased (+0.5%) to 69.497% when pulling 6075498 on Tyriar:207_selection_manager into 4b72791 on sourcelair:master.