HaikuDepot: Shutdown Handling

Improvements to the shutdown handling mechanics so that
if there is an install etc... happening when the
application is quit that it will wait until the process
is complete before actually terminating.

Change-Id: I8d3c4fbd9de0abc9382d55f0a6955b7f63a36637
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4322
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>