[sugar] Sugar proposals for sharing by invitation

[Morgan Collett]

For Trial 3, we are working on sharing by invitation, where the activity
is not advertised publicly.

I am changing the PS API's call for public sharing from ShareActivity to
AdvertiseActivity, and adding InviteActivity for the by-invitation-only
sharing.

sugar.activity.activity.Activity.share() calls
self._pservice.share_activity(self) which is
sugar.presence.presenceservice.PresenceService.share_activity().

I plan to make this share() call to sugar.presence just grow a
private=True/False parameter to control the advertise vs by invitation
mode - or should I make separate methods in sugar.activity to make the
PS API change explicit?

Regards
Morgan