agl-shell.xml: Introduce a new interface

agl-shell.xml: Introduce a new interface

In order to allow another other client bind to agl_shell interface, this
introduces a new specific interface which the client would bind to
first, issue a doas_shell_client request, wait for a response, and
proceeed further if the event received was successful. Afterwards, the
client can bind to agl_shell protocol (and assuming it got 'bound_ok'
event back) can further use the agl_shell protocol as it happens with
the shell client.

This approach avoids adding a new protocol interface and instead re-uses
the same interface, with the note that the shell client is still in
charge of handling background and panels.

Bug-AGL: SPEC-4503
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: Iac1d840a5f917b2a92fdfbdcdc583144d3942a1c