Copy mode 1.0

[daonb]

When the user scrolls up, search or select text the pane enters copy-mode. Upon entering two things should happen:

• a new indicator is displayed to the left of the network indicator
• the search bar is displayed

The search bar arrow buttons should work, the input have a yellow border and an "X" button should be added to exit copy-mode.

Scrolling all the way down should not exit copy mode.

Splitting shouldn't work, but the following keys should:

• q to quit copy mode
• space to begin selection
• escape to clear selection
• enter to copy selection to the clipboard and quit
• jhkl - for movments

[akariv]

I've implemented an initial version of Copy mode in #147
It depends on a change I've made in xtermjs - xtermjs/xterm.js#3274, which allows styling a part of the terminal without having to select or change its contents.

While we could use the above code with a fork of xtermjs, it's preferable to use the main branch of xtermjs and to incorporate whatever changes we need there (and not use a fork, which is not maintained in the long run).

For that, I've prepared a new pull request, implementing a 'Decorations API', which provides a new capability to 'decorate' the terminal with styles, widgets and stuff - obviously much more than what we need but that was what xtermjs developers had in mind.

I did some initial work there - xtermjs/xterm.js#3302 - which wasn't finished. There are some code review comments there, as well as some conflicting design requirements 😄 . Once that API is done and merged, we could re-implement the copy mode pull request to use it instead of our own patch.

@daonb