This gets the basic mechanics of the drop portion of DnD working on the
macOS backend. You can drag, for example, from TextEdit into GNOME
Text Editor when using the macOS backend.
Other content formats are supported, and match what is currently
supported by the clipboard backend as the implementation to read
from the pasteboard is shared.
Currently, we look up the GdkDrag for the new GdkDrop. However,
nothing is stashing the drag away for further lookup. More work is
needed on GdkMacosDrag for that to be doable.
