styling: TreeWidget Highlighted nodes full display

[FernandoAscencio]

What it does

There is a display discrepancy on selected nodes between Theia and VS Code, as seen below:

VS Code

Theia

This commit fixes that discrepancy in display. As seen below:

This Commit

Notes

I have added some extra code handling to keep the current display of Theia. However, as seen in VS Code, the left margin/padding for the Search-In-Workspace and Source-Control trees is no different from the Explorer and Reference trees. As such, if the VS Code tree alignment is preferable, it would also reduce the number of lines added.

How to test

1. Run example Theia
2. Do a search and modify a file
3. Check that the selection (both $\textcolor{blue}{\textsf{on focus}}$ and $\textcolor{gray}{\textsf{out of focus}}$) highlights the entire row instead of leaving a space.

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

[msujew]

Looks good to me. I couldn't see any regressions and the changes fix the issues at hand 👍

[vince-fugnitto]

Overall the changes look much better when selecting or highlighting nodes in trees.
I only have a minor comment.