Encapsulate Electron Window Logic - Allow Exit during Load

[colin-grant-work]

What it does

Fixes #10586
This PR is a pretty big intervention into the logic of the ElectronMainApplication. The narrow goal is to track whether a window on the frontend is in a state to handle a close request so that windows that aren't ready to handle the close request can be closed immediately. The larger goal is to begin to extract the logic of handling Theia windows so that we can develop an abstraction that will work for both Electron and browser with an eye to doing things like running cleanup when the last window is closed. There are much smaller interventions that can address the narrow goal, so if there isn't an appetite for this size of refactor, that's fine, and I'm happy to reduce it to the minimum necessary to ensure that close gets handled correctly.

How to test

It would be nice to write automated tests for some of this logic, but I haven't interacted with our Electron-specific test system much. Any pointers are appreciated.

1. Add some code that will prevent the frontend from ever achieving a ready state.

e.g. add a return to FrontendApplication#start that terminates it early.

2. Run the application and try to close the window.
3. It should close.

---

1. Let the application get ready normally.
2. Dirty a file, or do something else that would generate a close prompt.
3. Observe that the prompt is displayed correctly and the application closes or doesn't close correctly
4. Change the window.titleBarStyle (if not on Mac) preference and observe that the application restarts correctly.

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

[vince-fugnitto]

Wrong pull-request :)

[msujew]

The changes are looking really good to me! After applying the fix from #10597 I was able to successfully execute all testing steps:

• Shutting off the application before it reaches its ready state is possible
• Different configurations of the application.confirmOnExit preference, dirty editors, etc. work correctly when trying to exit the application
• Commands such as Configure Display Language or Reset Workbench Layout still ask for confirmation for a reload
• Switching between different window.titleBarStyle on different windows and reloading the windows continues to work correctly

[colin-grant-work]

Thanks, @msujew. I'll merge this after the release.