protocol/agl-shell.xml: Add an app_state event -- app state notification

This protocol bump will notify the client binding to the agl-shell protocol
when a particular application changed its state.

This includes four (4) different events:

- started
- terminated
- activated
- deactivated

This should allow orchestrating start-up with activation as we don't
really know when it would be the proper time to activate an application
when starting up (for the first time). A started event will notify the
shell client we it can do that. These events are not sticky such that
the shell would be responsabile for keep  track of the state, if it
wants to.

Bug-AGL: SPEC-4528
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: Id162874fa68946bb9b1db9fa356dd617a0db9eb7