fix(developer): handle shutdown cleanup more cleanly

Fixes #4757.
Fixes #6928.

Three separate ways shutdown is more robust:

1. Destroy main form before web server -- this ensures that all main
   form routines have deregistered app sources from web server.
2. In some situations, `FormClose` can be skipped by shutdown routines,
   for example, if you call `Application.Terminate`. This change ensures
   that we get a second chance to cleanup in the form destructor with
   `DoCloseCleanup`.
3. In the Online Update Check, instead of `Application.Terminate`, we
   now use `Application.MainForm.Close`, which is much more polite, as
   it ensures that changed files are saved, etc, and does proper
   cleanup. This particular change may make the other two somewhat
   irrelevant but they are still sensible as they will cover us for
   other potential form destruction sequences.