Copy GStreamer.framework in the .app on macOS
We need to improve the way we package the app on macOS.
http://doc.qt.io/qt-5/osx-deployment.html
#299 might make this obsolete
Related to #233
Made some progress on this today but it is really hard.
Will be invalidated if we get done with #379
Investigate https://github.com/GStreamer/cerbero
Tried to follow these tips, but they are not quite clear: https://stackoverflow.com/questions/36285647/how-to-link-qt-api-gstreamer-on-qt-creator-macos
might be fixed by #434