Add a new code for drag & drop that's non blocking.
Becuase of the usage of the QTabbar, a QDrag operation
would cause artifacts in the image because it would block
in the middle of a animation.
Fake the drag & drop operation using QMousePress/Move/Release.
The good thing is that the new code is smaller and easier to
understand than the old one, and it also fixes around 8 crashes.
Ready to Review.