Quick click leaves drag template ghost that won't disappear

[robbiehinch]

Added mouse button down guard around DragEnter

Please check the logging (now removed) added in commit 80d9ac5

Doing a fast click, drag, release can make the drag template stay open indefinitely. It's because the DragEnter event handler can be called after the DragSourceOnQueryContinueDrag event, but there are no mouse buttons depressed when it is called:

DragSourceOnMouseMove Left Pressed
_draginProgress true
DoDragSourceMove setting preview
UpdatePreviewPresenter ->
setting template
DragSourceOnQueryContinueDrag setting nulls
DropTargetOnDragEnter Copy, Move None
DragEnter
DropTargetOnDragOver setting preview
UpdatePreviewPresenter ->
setting template
UpdatePreviewPresenter ->
visual source not null
Drag cancelled
DragDropOperationFinished None
_draginProgress false
DragLeave

[punker76]

I think this is fixed by #497