xwayland: Implement X11-to-wayland DnD

When DnD is started from an X11 client, mutter now sets up an special
grab that 1) Ensures the drag source keeps receiving events, and 2)
Moves an internal X Window over wayland clients as soon as the pointer
enters over these.

That window will act as the X-side peer for the currently focused
wayland client, and will transform XdndEnter/Position/Leave/Drop
messages into wayland actions. If DnD happens between X11 clients,
the window will be moved away and unmapped, to let these operate as
usual.

https://bugzilla.gnome.org/show_bug.cgi?id=738312