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 a crash.
The bad thing is that it's ignoring when you have multiple tabs in the same window, it's always adding it on the first one that you have,
Also it's always adding them in the last position of the tab, I'll update this review as soon as I have those features fixed.
But please take a look, it already works for the majority of cases.