Implemented VectorDataPlugValueWidget.setHighlighted().

This makes it easier to see that you're about to accidentally drag and drop the wrong thing - the root cause of which will be fixed in upcoming commits.
Stopped PlugValueWidgets accepting drags from themselves.

This was causing trouble for users who were accidentally dragging and dropping a single line from a PathVectorDataPlugValueWidget onto itself, thus removing all the other lines. This addresses the underlying problem which motivated #593.
Added drag start threshold.

The Widget.dragBeginSignal() is now only emitted if the mouse has been moved a certain distance since being clicked. This reduces accidental drags.

Fixes #593.
Merge pull request #606 from johnhaddon/dragDropImprovements

Drag drop improvements