The selection is supposed to be sent to the DataDeviceInterface just
before getting keyboard focus. In order to do that the SeatInterface
keeps track of the DataDeviceInterface which is the current selection
and the DataDeviceInterface of the focused keyboard client.
SeatInterface friends DataDeviceManagerInterface so that the latter
can register each created DataDevice for the SeatInterface.
The Resource base class is supposed to be used by all interface
classes which get created for a wl_resource.
Most interface classes are adjusted, but there are some exceptions:
* BufferInterface: is different as the wl_resource is already created
* PointerInterface and KeyboardInterface: those two need changes, the
implementation differs from all other interface implementations.
Only selection part is implemented, drag'n'drop still needs to be
implemented.
Unit test is not properly testing whether the data can be transferred.
This needs some better architecture to have multiple processes which
perform the source and target part.
