Set focus to the next top-level window when close the main window

[yuya-oc]

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Summary
Set focus to the next top-level window when the main window has been closed.

Issue link
#430, #431

Test Cases

#430

1. Open Mattermost and another application window.
2. Activate Mattermost window.
3. Close Mattermost window.
4. Another window should get focus.

#431

1. Open Mattermost and another application window.
2. Activate Mattermost window.
3. Close Mattermost.
4. Show the classic Alt+Tab switcher. (See [Help Wanted] Mattermost desktop app icon appears in the "classic" Alt+Tab switcher list after app window is closed #431)
5. Mattermost should not appear on the switcher.

Additional Notes
Testable apps: https://circleci.com/gh/yuya-oc/desktop/187#artifacts

[jasonblais]

Works great, thanks @yuya-oc!

Tested both cases above. Also confirmed that after minimizing the app, the next top-level app got focus.

/ cc @nmatt the bug fixes for #430 and #431 should be resolved on the next desktop release (v3.7)