add quit label as the common quit process

Now, a user can execute label "confirm_quit" before quits ren'py, but it's called only by the close button.
It's not called by the Quit action or the quit of config.game_menu or config.main_menu.
I added the special label "quit" which is called by all processes when quit ren'py if it exists.

Issue: if confirm_quit or _compat_confirm_quit exists, this label is not called  by the close button.
Should I leave them for compatibility?