Introduce xdg-foreign protocol

xdg-foreign is a protocol meant to enable setting up inter surface
relationships across clients. Potential use cases are out-of-process
dialogs, such as file dialogs, meant to be used by sandboxed processes
that may not have the access it needs to implement such dialogs.

It works by enabling a client to export a surface, creating a handle
for the exported surface. The handle, in form of a unique string, may
be shared in some way with other clients (for example the provider of
the file dialog) which can then import the exported surface.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Mike Blumenkrantz <zmike@osg.samsung.com>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>