TelepathyClient: use polari for IRC channels

polari is the GNOME app for IRC, empathy is for everything else
So prefer polari to empathy for IRC channels. We don't need
to check that either exists (even though polari is not a core
app) because mission control tries every handler if the preferred
one fails.
Depends on bug 745418 for polari to be mission control activatable.

https://bugzilla.gnome.org/show_bug.cgi?id=745431