[problems] add a preference to control tree problem decorators

[vince-fugnitto]

What it does

Fixes #6019

• added a new preference to control whether to enable/disable
  problem decorators (diagnostic markers) in tree widgets.
• updated the problems.decorations.tabbar.enabled preference name.

How to test

• open a workspace
• open a file and create an error
• the explorer tree decoration should be enabled for the file
• open the preferences widget, and disable the preference problems.decorations.enabled
• the explorer decorations should be turned off
• re-enable the preference problems.decorations.enabled
• the explorer decorations should be turned on

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

Signed-off-by: Vincent Fugnitto [email protected]

[fangnx]

Works well! A very useful preference.

[akosyakov]

code-wise looks good

[akosyakov]

@vince-fugnitto Could we nominate @fangnx as a committer? He did quite complex changes already and his review should be counted.

[vince-fugnitto]

@vince-fugnitto Could we nominate @fangnx as a committer? He did quite complex changes already and his review should be counted.

He did in fact complete very nice complex changes that improved the app as a whole :)
Unfortunately, @fangnx's 4-month internship has just completed (last Friday) and he will be returning to school full-time so I wasn't 100% sure about nomination since he wouldn't be as active anymore.

cc @marcdumais-work