Getting the object associated with the particular wl_resource is not
difficult, but it involves a pretty reasonable amount of boilerplate
code.
This change, introduces a helper function with an intend to reduce the
amount of boilerplate code.
It can be used as resource_cast<const ObjectPrivate *>(resource) or
just simply resource_cast<ObjectPrivate *>(resource).
This was done mostly because I wanted to get rid of the Resource
dependency in AbstractDataSource so I can make our xwl bridge direct,
but this also fixes up some issues with object lifespan present in the
previous version and keeps all our clipboard code in-line.
SeatInterface cleans up when a data source goes away. However the
cleanup also makes use of metaobjects so needs to run before the leaf
class destructor is run.
All other AbstractDataSource objects emit the unbound signal, which
SeatInterface also connects to do a earlier cleanup. Otherwise we get a
crash if a data control source replaces itself.
