Use tail decorations for problems in TreeWidget

[colin-grant-work]

What it does

FIxes: #3819

With an eye to making it easier to reuse code in plugin tree views (see this PR), this PR adjusts our current problem tree decorator to use tail decorations rather than icon overlays. That is more consistent with VSCode and also carries over into dot decorations of parents implemented in this PR.

The tabbar decorator also uses icon decorations for problems, which is not consistent with VSCode either, but is a completely separate subsystem, and currently the tabbar renderers don't support tail decorations.

How to test

1. Open the navigator
2. Create problems in the workspace
3. Observe that files with problems are decorated with the number of problems in the file in the color appropriate to the highest-severity problem.
4. Observe that the parent folders receive a dot decoration of the appropriate color.

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 for me 👍

There are still some key differences with vscode that we should probably align with if we feel that it represents a better behavior, namely:

• the priority or ordering of the tail-decorations
• the fact that the decoration with the highest priority or severity takes precedence over the entire decoration (we may argue that this limits the decorations and feel as though our approach is better)

theia

vscode:

[vince-fugnitto]

I'm happy with the changes 👍
The improvements I identified are general and should be handled when or if we revisit how decorations work and how can simplify them.

[msujew]

The changes are looking good to me as well 👍