Remove redundant QObject:: prefix from some tr()

The class in which these tr() are called already inherit from QObject
so it can be called directly without using the QObject:: prefix.

This has the effect of changing the translation context to a more
meaningful one...
Add some new strings in the (Common) context.

At the same time, fix one of the strings in another class so that it's
written in the same way...
Miscellaneous translation fixes

Add some more strings in the (Common) context.

While there, make tr() usable directly in that class.
MythGallery translation context changes
Merge branch 'master' of code.mythtv.org:mythtv