TpAccountManager: tighten up guarantees about factories
TpAccountManager: tighten up guarantees about factories

It always has a factory, and if none is specified at construction,
the default is to use a TpAutomaticClientFactory. If we document
that, then client code won't need to mess about with
tp_account_manager_set_default() unless it has special requirements,
namely: either it has a factory of its own with more features and/or
subclasses, or a TpSimpleClientFactory is sufficient and low-overhead
is important.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=54061
Reviewed-by: Guillaume Desmottes <f16649c7afcd84e3350287183c4ed23cb1aea57c@collabora.co.uk>

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