TpBaseConnection: improve sequence of events for channel factories
TpBaseConnection: improve sequence of events for channel factories

We want to return from RequestChannel, then emit NewChannels, then
NewChannel.

Rationale:
* requester should get its answer first, so it can alter its handling of
  NewChannels accordingly
* clients that didn't introspect the Connection (so don't know whether
  to expect NewChannels) get the more informative signal first

Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib