Nepomuk support in Gwenview

Hello,

I just committed some work I did on Nepomuk support in Gwenview. Prior 
to this work, Nepomuk support was only present in the sidebar, now 
Nepomuk data is (will) be gathered by the SortedDirModel class, which 
allows nice things like showing thumbnail ratings and filtering the view 
to only show images with a rating greater than a given value.

It's not finished though: only rating is available for now, I plan to 
add back comment and tags soon.

Second important breakage: Due to Nepomuk instability lately I decided 
to abstract the way Gwenview interact with metadata. I introduced a 
MetaDataBackEnd class, which is used by the dir model class to gather 
metadata.
Right now I implemented a FakeMetaDataBackEnd, which returns random 
ratings based on the url. I also implemented a simplistic 
NepomukMetaDataBackEnd which, according to the API documentation, should 
work, but doesn't behave properly on my machine: it doesn't store or 
retrieve the rating information. I will try to figure this out with 
Nepomuk developer.

To choose between the various backends, I introduced a new CMake 
variable: GWENVIEW_METADATA_BACKEND. By default it's set to "None", 
which won't build any backend, but you can set it to "Fake" or "Nepomuk" 
to build either backend.

Feel free to give it a try.

Aurélien

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