tree indent guidelines support

[akosyakov]

What it does

• Fixes [ui/ux][tree] Help the tree indentation with vertical lines, mimic VS Code #6586:
  • Add preference to set 'workbench.tree.renderIndentGuides' to 'onHover' (default), 'none' or 'always'
  • When nodes are selected, indent guidelines are rendered for all the sibling nodes
  • When parent node is selected, indent guidelines are rendered for all the child nodes
  • When hovering over a tree, indent guidelines are displayed for all expanded nodes
  • Selecting multiple nodes works in a similar fashion

How to test

• Set the workspace preference to 'onHover', 'none', 'always' and the rendering of the indent guidelines should change accordingly.
• In different locations in Theia where the tree is being used (for example, in navigator, search in workspace, outline, problems-view and scm (list mode and tree mode)), select one/more nodes to test if the indent guidelines are rendered correctly.

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

The changes work well, I verified with multiple trees in the application 👍
The only minor issue is the styling of the indent guideline is bit off given the tree (the indent guideline in respect to the collapse icon):

Explorer:

Problems:

Search:

I do think this is a minor issue however and something we can fix in the future if necessary.

[akosyakov]

Search looks off indeed, but other don't much different compare to VS Code.

[akosyakov]

For search we can remove custom styling of the expand toggle to align with other trees then it looks like:

Should we? For comparison in VS Code:

[vince-fugnitto]

@akosyakov I don't think it's necessary, at least not for the moment (should not block the pull-request).
The issue is that some trees define their own custom styling which is different than the base tree which is what @kaiyue0329 was trying to fix with adding new props, but it makes the changes complex. I'd be fine with keeping the approach simple and if we ever update we can do so later on.

[akosyakov]

@vince-fugnitto I would rather looking into normalising such trees with defaults and removing custom styling. It seems to be also only one case. I've checked other trees like scm, problems, outlines and could not understand why we need to change their styling. They looked aligned with the default implementation without changes.

[vince-fugnitto]

@vince-fugnitto I would rather looking into normalising such trees with defaults and removing custom styling.

@akosyakov I agree as well, it'd be best if all trees were normalized this way and their behavior was consistent.

[akosyakov]

merging tomorrow morning if no objections

[Anasshahidd21]

The changes look pretty good to me. Tested on multiple views such as navigator, scm, problems, etc,