editor: combine left gutter context menus

[cdamus]

What it does

For compatibility with the VS Code behaviour of the "editor/lineNumber/context" menu contribution point, combines the context menus created by

• debug extension in the area to the left of the line numbers and
• editor extension in the line numbers, themselves

into one context menu, managed by the editor extension, opened either from the line numbers or from the gutter area to their left.

Fixes #12688

How to test

1. Clone the menu_bug_3 branch of the sample repository provided by the OP of VS Code menu contribution to "editor/lineNumber/context" doesn't behave as in VS Code #12688 as described in that issue and build it.
2. Link the myext extension from step 1 into your plugins/ directory.
3. Launch Theia.
4. Open a code editor on some TS file or something and right click anywhere in the left gutter of the editor where there is some line of text. You should see something like this:
   
5. Observe that the "Hello, World!" action shows the message toast as expected.
6. Verify also that the menu actions to create and remove breakpoints work as expected whether the context menu was summoned by right-clicking on an actual line number or to the left of it.
7. Verify further that simply clicking in the gutter area to the left of a line number, without summoning the context menu, still toggles line breakpoints.

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

[jcortell68]

@cdamus Thanks for fixing this!

[martin-fleck-at]

Change looks good to me and the behavior is now as in VS Code where all actions are visible and the debug actions are generated on the fly. Great work, Christian! Thank you!

[martin-fleck-at]

@colin-grant-work @msujew: Would any of you like to have a look at this as well? I saw you guys did some work in this area.

[msujew]

Looks good to me as well 👍