Add PurpleTypingState and add it as a property to PurpleConversationMember

This is a simple API to allow use to track typing state for contacts for any
conversation type. It also has built-in timeout support to reset a state back
to the PURPLE_TYPING_STATE_NONE if not other state changes have happened.

Testing Done:
Ran the unit tests and verified the docs.

Bugs closed: PIDGIN-17763

Reviewed at https://reviews.imfreedom.org/r/2288/