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>