Qt5 adaptions

After applying the changes from [1], Cockatrice should now also work with Qt5 without losing Qt4 compatibility. I actually don't have Qt5, so I could not test it.

[1] http://qt-project.org/wiki/Transition_from_Qt_4.x_to_Qt5
Probably I need to change this, because there are QTextCodec::codecForName("UTF-8") instances in the code, which means that the default won't be Latin1 but Unicode.