Drag and Drop JPG/PNG from explorer window to Material map slot

[^Lele^]

The update to QT speeds up UI redrawing tenfold.
We had to move away from the old controls ASAP.

We can't unfortunately revert the changes (we'd be lynched by a few of our users, i'd wager.), and will have to work with AD to solve the standing issues.
You can stay in 5.1 should you need the feature more than the UI speed, though (well, you'd also miss out on other things, ofc.).