Reuse same DebugSessionWidget for all debug sessions.

[colin-grant-work]

What it does

• Depends on Ensure that the Debug API is working with the new RPC #11268

Fixes #9930 by using the same DebugSessionWidget for all debug sessions. It removes a fair bit of code that assumes that we will open many DebugSessionWidgets, which could break downstream applications that exploit that assumption.

How to test

1. Start more than one debug session (e.g. start one using the Run Mocha test configuration, and another on a basic JS file using the dynamic session)
2. Switch between sessions.
3. Assert that switching between sessions updates widgets appropriately (or at least as on master) and that the debug toolbar controls the currently selected session.
4. Move some of the children of the DebugSessionWidget into other view containers, and assert that (3) is still true.
5. After moving some of the children of the DebugSessionWidget, restart the application.
6. Assert that the moved children remain where they were before the restart.

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

[msujew]

That's a lot of removed code :D

Anyway, I couldn't notice any regressions from master and the improved behavior works correctly

• Moving a debug view part to another view container works as expected
• Changing to another debug session (in one container) updates the call stack view (in another container) correctly
• Moved view parts stay where they are supposed to be after reloading the app

A minor thing which also doesn't work on master currently is that switching between debug sessions doesn't update the select component (but correctly updates the debug repl):

This is likely out of scope for this PR, but I just wanted to write it down somewhere.

[colin-grant-work]

@thegecko, I know that debugging is a topic of interest to ARM. Is the ability to open multiple sessions simultaneously in different areas important to you?

[msujew]

Alright, looks good 👍

[thegecko]

@thegecko, I know that debugging is a topic of interest to ARM. Is the ability to open multiple sessions simultaneously in different areas important to you?

Thanks for the heads up. At the moment, we don't require support for running multiple debuggers with separate UI panels.
As long as this works (as confirmed by others above) and this mimics the VS Code workflow then I'm happy to get this merged.

In fact it makes my open PR a little simpler (although it will need rewriting). I'll revisit that when this is merged:

#10748

[thegecko]

Code looks good and tested the debugger still works as expected