Catch schema change event in order to have actual representation of user preferences

[vzhukovs]

What it does

This changes proposal fixes the problem, when user preference tree wasn't respect preference schema change event. The problem was in initializer code fragment that collect combined preference scheme during tree create. Here is a sample of plugin, that has definition of preference in package.json. The flow of representing problem. HostedPluginSupport initialize and calls load function and then doLoad where loads contribution and handle it, calling this function. Then it takes contribution configuration and operates with scheme, setting it up into preference provider where preference changed event fires and nothing appears. So there is a reason to subscribe preference tree listen to this kind of event to be able to update the representation of preference tree when schema changes.

There is a linked issue for this problem: eclipse-che/che#15001

Signed-off-by: Vladyslav Zhukovskyi vzhukovs@redhat.com

How to test

Clone this sample plugin, build it and place in theia/plugin directory and start theia. In master branch there is a problem, when user refreshes the page where opened preference editor and after refresh configuration from the custom contribution disappears.

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

[akosyakov]

We should test that selection and expansion are preserved when schema is changed.

[akosyakov]

@vzhukovskii please squash follow-up fixes for code clean ups and issues introduced by a PR itself in the original commit: https://github.com/eclipse-theia/theia/blob/master/doc/pull-requests.md#checklist-commit-history

[RomanNikitenko]

Tested using sample plugin from How to test section - the issue is fixed for me: plugin preferences are displayed after page refreshing.

[RomanNikitenko]

btw, from my point of view it makes sense to check this fix for #6467
@lmcbout

[vzhukovs]

Changes squashed into single commit.

[akosyakov]

i think you can merge if noone objects and CI is green