workspace: Stop using WindowPreview.realWindow in the overview

workspace: Stop using WindowPreview.realWindow in the overview

We can simply get the MetaWindowActor by calling
MetaWindow.get_compositor_private(), so stop accessing the realWindow
property of WindowPreview. For this we also have to make _isMyWindow()
and _isOverviewWindow() take a MetaWindow as an argument instead of a
MetaWindowActor.

Since the WorkspacesThumbnails are also drop targets for WindowPreviews
and their WindowClones also have the public metaWindow property, switch
to using the metaWindow property there, too.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1307