Allow opening changes for files associated with custom editors

[pisv]

What it does

Allows the user to visually compare two versions of a file that is associated with a custom editor. This is done by opening both versions of the file simultaneously in a side-by-side editor, with each version being opened in a separate custom editor contained in the side-by-side editor.

For example, this feature can be used to help spot the differences between two versions of an image file associated with the custom editor contributed by the VS Code media-preview plugin, as illustrated with the following screenshot:

More screenshots illustrating this feature being used with other custom editors:

This PR contains a series of individual commits that work together to make it possible for the CustomEditorOpener to open a diff-uri in a side-by-side editor containing the corresponding CustomEditors. While the main functionality is contributed by the latest commit, the earlier commits provide various fixes and enhancements required to make it work.

Here is the sequence of commits:

• Fix loading of webview resources that depend on query params

  Some resource url (notably git) use the query part of the url to store additional information. The query part of the url was incorrectly being dropped while attempting to load these resources inside of webviews.

• Fix Error: Unknown Webview messages in the log

• Fix design and implementation issues surrounding CustomEditorOpener

  This commit ensures that the promise returned by CustomEditorOpener.open will only resolve to a properly initialized and opened CustomEditorWidget. In particular, it ensures that the widget is opened according to the specified WidgetOpenerOptions, including widgetOptions.ref and mode.

  Essentially, it revises the work done in Fix #9670: Custom editor plugin activation called 3 times when double click quickly #9671 and Fix the widget options when opening a VSCode Custom Editor #10580.

• Restore custom editors as part of layout

  Fixes an incorrect assumption that a custom editor cannot be restored if no WebviewPanelSerializer is registered for its view type. (Actually, custom editors are created and restored using a custom editor provider.)

  Also, ensures that CustomEditorWidget.modelRef satisfies the shape for the CustomEditorWidget defined in editor.ts and cannot return undefined. (However, CustomEditorWidget.modelRef.object can be undefined until the custom editor is resolved.)

  Fixes custom-editor: restore editor as part of layout #10787

• Fix a race condition when file system provider is activated

  When file system provider is activated, wait until it is registered.

• git: add support for custom editors

  • Uses OpenerService instead of EditorManager to open editors

  • Contributes a FileSystemProvider for git-resources

  • Fixes an issue with getting blob contents

• custom editor: open a diff-uri in a side-by-side editor

  CustomEditorOpener is now able to open a diff-uri in a side-by-side editor, which contains the corresponding CustomEditors.

  Fixes [Diff] Images should open in preview #9079

How to test

There are various configurations that can be used for testing this feature:

• Windows vs. Linux vs. macOS

• Browser vs. Electron

• vscode.git vs. (now deprecated) @theia/git

This feature can be tested with any custom editor.

A basic test scenario involves making a change to a file associated with a custom editor (e.g., .jpg, .png, .pawDraw, or .cscratch) and then opening changes in a side-by-side editor by clicking on the file in the Source Control view.

Since this PR also fixes an issue with custom editors not being restored as part of layout, it is worth checking out that custom editors are now correctly restored, both standalone and as part of side-by-side editors.

Also, it is important to verify that there are no regressions, especially that standalone custom editors can still be opened and work as expected.

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

[pisv]

All checks are now green, and this PR is eagerly waiting for a review :-)

[msujew]

FYI, I'll likely take a look at this, but don't have time for it before the upcoming release. Probably next week.

[pisv]

@msujew Good to know! Thank you. I'd really appreciate your review.

[msujew]

Sorry for the delay, but unexpected deadlines came up. Will have time to review starting next week :)

[pisv]

No worries :) That'd be great! Thanks!

[pisv]

(I've resolved a merge conflict and retested everything in preparation to the upcoming review.)