feat(window): show drag overlay

Implements a DnD overlay, thus closing #59 (closed). This MR originally started as a fix for the problems with https://gitlab.gnome.org/GNOME/Incubator/loupe/-/merge_requests/232, but quickly turned into an implementation based on Amberol's DragOverlay.

If the properties view is open, the overlay will only be applied to the image view.