Add new events dedicated to invitation handling

[vxgmichel]

Those new events should be aimed at helping with the synchronization of claimers and greeters.

This is especially useful for the greeting of a claimer during a user invitation. Since the user can be greeted by any administrator, an event could be generated in order to notify that a given claimer is waiting for an administrator willing to invite them. If a claimer chooses to wait for a specific greeter, then another event could be sent to notify the user that a claimer is specifically waiting for them. Another useful event would to signify that a claimer has been joined by a given greeter, for instance to notify other administrators that the invitation is already being taking care of.

To sum it up, there could be:

• a ClaimerConnected event
• a ClaimerWaitingForPeer event
• a ClaimerJoinedGreeter event