We also propagate the PurpleProtocolActions::actions-changed signal from the
PurpleProtocolManager::account-actions-changed signal if the protocol implements
the PurpleProtocolActions interface.

Build the docs and ran the unit tests.
I also put a temporary action in the demo protocol plugin and called purple_protocol_actions_changed after the demo protocol connected. I will have another pull request that makes this a permenent action, but I want to convert the demo protocol plugin to subclass PurpleConnection before doing so as that will be accounting of everything much easier.