chat-manager: define CHAT_MANAGER_PATH

https://bugzilla.gnome.org/show_bug.cgi?id=723766
prefix Telepathy bus names with TP_

So it's clearer that we are referring the Telepathy bus name and not the
GtkApplication one.

https://bugzilla.gnome.org/show_bug.cgi?id=723766
add empathy-bus-names.h

It's convenient to have a single file containing all the D-Bus names so a
component can easily call a method on another one.

Also rename from DBUS_NAME to BUS_NAME to stay coherent with the TP_BUS_NAME.

https://bugzilla.gnome.org/show_bug.cgi?id=723766