Serious side effect of #6299 (silencing salut so gabble can connect)

[Morgan Collett]

We're testing patches to Presence Service to not start salut (or stop
it) for a while to give gabble a chance to connect to the schoolserver.

However, Daf came across what was a very minor problem which becomes
more serious in light of this change.

Many activities are calling PS get_preferred_connection() to interact
directly with the appropriate Telepathy Connection Manager, which was
required in the past before we expanded Presence Service's management of
setting up channels for activities.

However, during the period when we stop salut to let gabble try to
connect, this call fails as there is no running plugin in PS. If an
activity is launched during this time (and there's no particular UI to
show this other than no buddies in mesh view) and it makes this call in
__init__ as most of them do, then it will crash with a gray screen.

This affects: Calculate, Chat, Pippy, Record, Web and Write (of the
activities we bundle) and potentially other non-bundled activities.

Our options are:

(a) Touch all these activities now and port them to the newer cleaner
API offered by PS/Sugar
(b) Don't do #6299 for Update.1, but do it and (a) for Update1.1
(c) Find some way for the call to get_preferred_connection to fail
gracefully (We can't think of one so far)
(d) Make a UI change to let the children know not to launch activities
during this time period
(e) ???

Morgan