Moving KFIleMetadata into KDE SC

[Àlex Fiestas]

· I would add the url and mimetype in the cto, plugins should not be able to 
change it anyway.
· Documentation needs improvement, specially ExtractionResult (making it clear 
that you should inherit it in order to use the lib).
· ExtratorPlugin::mimetypes is pure virtual yet it has an implementation
· Add context to i18n in typeinfo
Also you might want to use Qt translation libs so kfilemetadata becomes a Qt 
only lib, that might help other desktops/apps to use it (in kf5 you can use 
k18n if you want).

Once this is fix, yo get +1 from me !