Previously [GtkApplicationDelegate applicationShouldTerminate:]
triggered both StartupFSM::queue_finish and ArdourUI::queue_finish.
Both of which registered an idle-callback to call ArdourUI::finish.
In some startup flows, the entire loading process happens inside
StartupFSM::start(). In others, that call gets things moving but
we return from it and loading is not complete until later.
Deleting the StartupFSM while still inside the ::start() call
led to a use-after-free error. This new code will leak the StartupFSM
in some startup flows.
This currently only handles initial session load by dbl-click on
a session file. It also does not work if the user or new or
subject to a pre-release warning. These last two restrictions
may remain in place, although if a person's first exposure to
the program is "copy my session, download program, dbl-click
on the session file", then that's not so nice
