HaikuDepot : Refactor Screenshots

Rework the way in which the application deals
with screenshots so that there is a repository
of screenshots that is referenced from
identifiers on the package model and not that
the screenshot bitmaps are stored with the
model and not cleaned up.

Change-Id: I632def96852a518b3f6b1c8b81909de117bbbc54
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7296
Reviewed-by: Andrew Lindesay <apl@lindesay.co.nz>

HaikuDepot: Fix Crash on Shutdown

The MainWindow itself was a BReferencable via a listener
virtual class. It looks like the error has come from a
conflict between the MainWindow's own deletion and the
state of the reference count. This change moves the
listener / BReferencable out of the MainWindow so that
the lifecycle of the BReferenable can be managed
correctly.

A further problem here is that the new listener
was leaked from the MainWindow class on shutdown.
To resolve this problem requires a considerable
change to the "process coordinator" system.

Fixes #17689

Change-Id: I7230843ba05537015f4a597b4a616b96c6db3dde
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5285
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Andrew Lindesay <apl@lindesay.co.nz>

HaikuDepot: Background Processing

Unify the background processing in the
application to one system and refactor
the package actions. This won't fix
any specific current issues with this
area, but lays the ground for later
fixes.

Change-Id: I098b81e7afc4aeb883bddf1acfc01f7888134257
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3997
Reviewed-by: Andrew Lindesay <apl@lindesay.co.nz>