Add membership management to PurpleConversation

This is one of many steps to making PurpleConversation handle all conversation
types instead of being an abstract class. In modern chat protocols the only
distinction between different types of conversations is an enum property and
members can join and leave dms, group dms, and multiple user chats. As such
this change is necessary to facilitate that.

Testing Done:
Ran the unit tests under valgrind and built and verified the docs looked alright.

Bugs closed: PIDGIN-17768

Reviewed at https://reviews.imfreedom.org/r/2287/