Implement data device stuff for copy n paste / drag n drop

[malcolmstill]

The associated interfaces are:

• wl_data_offer
• wl_data_source
• wl_data_device
• wl_data_device_manager

We already have an empty implementation of wl_data_device_manager.

My current reading of wayland.xml makes me think we need a Seat resource, that like Output is not associated with any particular Client but is global to the compositor (and each wl_output container points to a shared Output so we can do something similar). There is a current assumption that there is only a single Seat which is why this hasn't come up before.

Associated with each Seat then is an option wl_data_device