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.
