Initial port of picard/* to Qt5

I'd like Picard to have proper VoiceOver/accessibility support on Mac OS X (we've gotten several support@ emails from users who cannot use Picard at all). This might even be a prerequisite for submitting Picard to the app store.

This requires porting Picard to Qt5. The carbon version of Qt4 actually had some accessibility features enabled, but it was gutted from the cocoa version (only to be mentioned in a footnote somewhere).

This will break things as far as plugins/packaging are concerned, so I would recommend bumping the major version of Picard to 2.0.

I haven't ported any of the plugins, but can work on that if this is merged.
I'm not aware of Picard being switched away from gettext, but I fixed the regex for converting these in https://github.com/musicbrainz/picard/commit/4e67e59bd4e55c568b07edb4da33e1e8fd4821a2