protocol: agl-shell-desktop: Send notification for application state change

protocol: agl-shell-desktop: Send notification for application state change

The events are sent straight after the activation took place. The state
changes are the surface role (for instance, pop-up) and the
activate/de-activate type of event. With that information there's also a
string type of data which can be used as easy way to forward data.

Note that this isn't the proper way for applications to
communicate with each other, but merely as a convenient way to pass data
from one application to another. In order to hang-off the data, the
'activate_app' request also got an additional argument which is relayed
back with the event.

Bug-AGL: SPEC-3269

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I0c9be86f6ff227b59271cac1c060563b5aac9b6c