Modernize PurpleProtocol

* 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.

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

Bugs closed: PIDGIN-17492

Reviewed at https://reviews.imfreedom.org/r/552/