x11: Make fallback window dragging use seat grabs
x11: Make fallback window dragging use seat grabs

Also, gdk_seat_ungrab() will undo the implicit grab when EWMH dragging
is used.

grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita