Single, plugin based akonadi-nepomukfeeder

This is a refactoring of the nepomukfeeder agents to a plugin based
architecture, so we can control the system load better in a single process.

With the new approach plugins can be written to index specific mimetypes.

Other changes include:
-No Strigi indexing code in the agent (this should go to the relevant plugin)
-No activity detection (the feeder shouldn't place any noticeable strain on the system anymore)
-Backport of a couple of changes which were applied to the kdepim copy
(error message, progress reporting, disable feeder config)
-The feeder will always only index what is in the local cache, and not download anything.