Move download management from frontends to rss_sqlite.py.

Move download management from frontends to rss_sqlite.py.

 - Move download logic to rss_sqlite.py:
  - Move thread management to jobmanager.py.
  - Queue jobs in rss_sqlite.py.
  - Manage per-feed DBus locks in rss_sqlite.py.
  - Get config settings in rss_sqlite.py on demand.

 - Make downloading more robust:
  - Only call non-thread-safe code (in particular, DBus and gconf
    functionality) from the main thread using mainthread.py.
  - Improve responsiveness of the frontend but yielding during CPU
    intense activities and only allowing a single slave to parse
    a feed at a time.
  - When downloads are canceled, do our best to quit cleanly and
    quickly.

 - Update frontends to use the new functionality.
  - Remove redundant code, in particular, download functionality.
  - Rework FeedingIt.py's download bar.