Bug 31105

+++ This bug was initially created as a clone of Bug #30088 +++

In Bug #30088, I wrote:

One thing this branch doesn't have is a client-side binding in TpProtocol,
which would have to parse the statuses from .manager files (secretly part of
TpConnectionManager if I remember correctly) and/or from D-Bus properties.

Straw man:

If supported, Presence is part of TP_PROTOCOL_FEATURE_CORE.

I don't know whether these method names should contain presence_ or not: it
makes their meaning more explicit, but makes them more verbose.

/* requires CORE, returns NULL if Protocol.I.Presence is unsupported
 * or CORE isn't ready */
GStrv tp_protocol_dup_[presence_]status_names (TpProtocol *self);

/* returns FALSE without error for unrecognised names */
gboolean tp_protocol_can_set_[presence_]status (TpProtocol *self,
    const gchar *status_name);

/* returns FALSE without error for unrecognised names */
gboolean tp_protocol_[presence_]status_has_message (TpProtocol *self,
    const gchar *status_name);

/* perhaps this is useful to have? */
GStrv tp_protocol_dup_settable_[presence_]status_names (TpProtocol *self);