CatalogueUI : Don't "steal" irrelevant drags

We were accepting any drag which provided StringVectorData, when we should only have been accepting those which originated in the image listing (because we are using the drag to reorder images). This meant we were accepting drags of paths from the HierarchyView, and then clobbering the custom pointer in `__pathListingDragLeave`. This gave people the impression that the drag was broken, when in fact you could still continue and drop elsewhere (despite the pointer indicating otherwise).

Also tweaked the drag move logic so we use exactly the same checks in both enter and move.

Changes.md

@@ -1,7 +1,10 @@
 1.4.15.x (relative to 1.4.15.2)
 ========
 
+Fixes
+-----
 
+- Catalogue : Fixed bug which "stole" drags that crossed the image listing but which were destined elsewhere, for instance a drag from the HierarchyView to a PathFilter in the GraphEditor.
 
 1.4.15.2 (relative to 1.4.15.1)
 ========

python/GafferImageUI/CatalogueUI.py

@@ -1079,7 +1079,7 @@ def __dropImage( self, eventData ) :
 
 	def __pathListingDragEnter( self, widget, event ) :
 
-		if isinstance( event.data, IECore.StringVectorData ) :
+		if event.sourceWidget is widget and isinstance( event.data, IECore.StringVectorData ) and event.data :
 			# Allow reordering of images
 			self.__moveToPath = None
 			self.__mergeGroupId += 1
@@ -1101,7 +1101,7 @@ def __pathListingDragLeave( self, widget, event ) :
 
 	def __pathListingDragMove( self, listing, event ) :
 
-		if not event.data or not isinstance( event.data, IECore.StringVectorData ) :
+		if not ( event.sourceWidget is listing and isinstance( event.data, IECore.StringVectorData ) and event.data ) :
 			return
 
 		targetPath = self.__pathListing.pathAt( event.line.p0 )