Defer creating new editor process in "Quit to Project List" just like in "Reload Current Project"

[Listwon]

Possible fix for #40968
Reload Current Project creates new editor process when all of the files used by editor are closed (as in FileAccess closed).
I was investigating the issue #40968 - why Quit to Project List was wiping my editor settings and the list of recently opened projects. I've noticed that my debug logs from closing files were mixed with new editor instance logs, meaning that in some cases they could collide in file locks.
Using the solution from Reload Current Project (deferring with OS::get_singleton()->set_restart_on_exit(true, args);) prevents it.

It has to be applied to 3.x too, originally I've encountered the #40968 issue in multiple 3.x versions (recently in 3.5.2 Mono).

Bugsquad edit:

• Fixes When quitting the editor to the project list, editor settings are lost #40968

[KoBeWi]

Looks like a safe fix. We could look into unifying the code in restart_editor() and RUN_PROJECT_MANAGER, because they are very similar. That's for later though.

[akien-mga]

Thanks!