Migrate PurpleProtocol to the G_DECLARE_DERIVABLE_TYPE and G_DEFINE_TYPE macros.
Rename purple_protocol_class_* to purple_protocol_*.
Move the class properties to instance properties and add accessors as necessary.

Ran locally and connected with bonjour, facebook, irc, and xmpp.  Verified that the other prpls are loading and visible in the account manager.