Fix build on firefox plugin

The pl-PL directory was recursed in Makefile, but no Makefile was
generated on that directory.
tracker-extract: Split generic gstreamer/libav extract rules

This is so audio, images and videos going through these modules get
more specific rdf:types than nfo:Media, as it is usually nfo:Media
subclasses which get flagged as tracker:notify. This fixes situations
where GraphUpdated wouldn't be emitted for files where it really
should.