Replace the account-actions-changed signal with an actions-changed signal on PurpleProtocolActions

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

Testing Done:
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.

Reviewed at https://reviews.imfreedom.org/r/2168/