Forwarding dispatch implementation

[Drakulix]

Draft while I didn't even crate a wayland-rs PR for the required changed. See Drakulix/wayland-rs@62a7ec5.

Example for xdg-activation (semi random pick).

This allows forwarding Dispatch implementation, e.g. to handle internal locks.

As a side effect this requires almost the opposite direction of what #1327 tries to accomplish. So far it looks like we need D to implement GlobalDispatch/Dispatch for this to work, I haven't been able to get this to compile with @PolyMeilex patches without introduces heaps of new generics.

For this to be usable we obviously need delegate_dispatch!/delegate_global_dispatch to optionally handle the new F parameter, which would get rid of at least one of the manual implementations in the example.

I would additionally like to add a forward_dispatch!-macro to get rid of the other.

Open for comments and further discussion. This would be very useful for cosmic-comp for a variety of reasons.

E.g. this enables internal locks for more fine-grained multi-threading (as can be seen in the example). This also allows to limit implementations of our *Handler-traits to parts of D. E.g. the SeatHandler could run on a subset of D, allowing set_grab and other to be called inside of the window-management code, where you might not have access to all of D.