Widget : Set DragDropEvent.sourceWidget from foreign drags

Our assumption was that such drags would always comes from outside of Gaffer - the foreign drag handling was originally added so that we could drag files from the system browser into the GraphEditor. In that case there is no source of information for `sourceWidget`.

But some folks prefer to code their Gaffer extensions in pure Qt, with a tiny shim to host them in GafferUI. In this case, we _can_ find a suitable `sourceWidget`, which we now do.

Changes.md

@@ -5,6 +5,7 @@ Fixes
 -----
 
 - GraphEditor : Fixed errors when dragging an unknown file type into the GraphEditor.
+- Widget : Fixed `event.sourceWidget` for DragDropEvents generated from a Qt native drag within the same Gaffer process. This will now reference the `GafferUI.Widget` that the Qt source widget belongs to, if any.
 
 1.4.15.2 (relative to 1.4.15.1)
 ========

python/GafferUI/Widget.py

@@ -1578,9 +1578,13 @@ def __foreignDragEnter( self, qObject, qEvent ) :
 			Widget._modifiers( qEvent.keyboardModifiers() ),
 		)
 		dragDropEvent.data = data
-		dragDropEvent.sourceWidget = None
 		dragDropEvent.destinationWidget = None
 
+		if isinstance( qEvent.source(), QtWidgets.QWidget ) :
+			dragDropEvent.sourceWidget = GafferUI.Widget._owner( qEvent.source() )
+		else :
+			dragDropEvent.sourceWidget = None
+
 		if widget._dragEnterSignal( widget, dragDropEvent ) :
 			qEvent.acceptProposedAction()
 			self.__foreignDragDropEvent = dragDropEvent