Offload all downloading logic to updatefeed.py.

Offload all downloading logic to updatefeed.py.

 - Change Woodchuck initialization to only process upcalls if started
   from updatefeed.

 - Add org.marcoz.feedingit.UpdateProgress.

 - In rss_sqlite.py, detect if we are running as updatefeed.py:
  - If so:
   - Perform downloads directly, and
   - Make the appropriate progress calls.
  - Otherwise:
   - Don't update feeds directly, instead, make the appropriate dbus
     calls to updatefeed.py (starting it as necessary).

 - updatedbus.py:
  - Remove get_lock and release_lock, which is no longer needed as
    updating is now centralized.
  - Add dbus methods Update and UpdateProgress.
  - Make methods UpdateAll and StopUpdate abstract.
  - Add function update_server_object to retrieve any UpdateServerObject
  - Ensure that there is only a single instance of UpdateServerObject.

 - update_feeds.py:
  - Move system idle detection from FeedingIt.py to here.
  - Add a '--daemon' option, which does not update all feeds on start.
  - Implement Update, UpdateAll and StopUpdate
  - Make update notifications as appropriate.
  - Update locking mechanism: grab the bus name without queueing.

 - FeedingIt.py:
  - Adapt progress logic appropriately.
  - Don't detect when the program is started only for an update (it
    isn't anymore)

 - FeedingIt:
  - Start updatefeeds.py with the --daemon option.