Fix memory leak in ApplicationShell#activateWidget

[colin-grant-work]

What it does

The ApplicationShell#activateWidget method awaits a Promise returned by ApplicationShell#waitForActivation, but that Promise is not guaranteed to be settled. In particular, if the widget in question fails to accept focus within 2 seconds, it will not be set as the ApplicationShell's activeWidget. Because of that, a call to activateWidget can hang indefinitely, and the Promise handlers retain references to the widget that was to be activated even if that widget is later closed.

This code ensures that the Promise returned by waitForActivation will settle so that activateWidget will run to completion.

How to test

1. Comment out the implementation of onActivateRequest in vsx-extensions-view-container.tsx.
2. Run the application and open the extensions view using the View menu.
3. In the console, see a message about the widget's failure to accept focus in a timely fashion.
4. Close the extensions view widget.
5. Open the dev tools and take a memory snapshot. Search for VSXExtensionsViewContainer
6. You should not see any retained references, and in particular, you should not see this as a retainer:

That chain indicates that the view container is being retained (in part) because of the promise in ApplicationShell#activateWidget.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

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