Fixed multiline selection issue in edit mode

[Jazzcort]

Description

The original behavior was only selecting the highlighted part and if there is nothing selected in the trailing line it still got seleced to the edit mode. This fix is to expand the selection start to the very beginning of first line and the selection end to the very end of the last line. In addtion, prevent the trailing line getting selecting if there is nothing in it.
Granite-Code/granite-code#42

Testing instructions

1. Partially select the first line and then Cmd-I, and check if the entire first line get selected in the editor and display correctly in the edit mode section.
2. Select empty trailing line and then Cmd-I, and check if the trailing line not get selected in the editor and not include in the display of the edit mode section.

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[owtaylor]

This looks like it will fix the problem I identified. I see a few style things that could be improved.

[owtaylor]

This would read better if it was:

if (selectionStart.line > selectionEnd.line || (selectionStart.line == selectionEnd.line && ...)) {
      selectionStart = editor.selection.end;
      selectionEnd = editor.selection.start;
} else {
    ...
}

[owtaylor]

What this comment should include is not what its doing but why, so more like:

// If the user selected onto a trailing line but didn't actually include any characters in it
// they don't want to include that line, so trim it off.

[owtaylor]

The should be some sort of comment here about what this is all about, would suggest:

// Edits are always of complete lines - we want to be consistent for that when we highlight,
// show the range in the sidebar, or send it to the model. To simplify, this, expand the range
// here to be complete lines.