The plural/paucal form support was not compatible with pylupdate4, it
didn't correctly parse the 3-argument calls to translate.
This fixes it, and updates the sources accordingly.
Added a RetranslateMixin. Since PyQT does not support automated language
changes of UI files (like the C++ QT does), this implements something
similar. It assumes that the UI file has the same name as the class, but
lowercase.
Added RetraslateMixin to the new blacklist and networkstatus interfaces.