Merging filtering capabilities in master

["Juan A. Suarez Romero" <jasuarez igalia com>]

Hello.


Today I've merged the work done by Guillaume to add filtering
capabilities to the sources.

So now, when performing an operation like search() or browse(), user can
filter the results:

* By type: for instance, returning only audio and video content, but not
images

* By key: for instance, returning all the medias with artist as
"Madonna"

* By range: similar to the previous one, but using ranges (for instance,
all songs with bitrate between 128 and 192 kbps).


Each plugin/source defines which filtering capabilities it provides, so
application can query them and act accordingly.

As example, right now only filesystem plugin is providing such
capabilities.

Finally, we have changed the test UI so we can now filtering by type all
the search results: select the filesystem source in the search combo,
and the audio/video/image check buttons will become active so user can
select the elements he needs when doing a search.

All credits go to Guillaume.

	J.A.