qtcollider: QView - do not block beginDrag if currentDrag is already set

Drag-and-drop operations initiated from outside sclang do not clear drag
data (i.e. currentDrag), so we have to trade off this sanity check for now.

Ultimately, things should be fixed by clearing currentDrag after external
DnD has finished. The issue is that there is no Qt event that would notify
about cancellation of DnD by the user. Maybe connecting to QMimeData
destruction signal would do.

An alternative would be to set and clear currentDrag at beginning and end
of every DnD event-handling method.