Add a get_prefix method to the PurpleProtocolActions interface

This make it much easier for protocol authors to keep track of things, but we
do fall back to using the id of the protocol for the prefix if get_prefix is
not implemented.

Testing Done:
Verified the fallback works, then changed the prefix in the demo protocol plugin and verified everything still functioned.

Reviewed at https://reviews.imfreedom.org/r/1547/