PurpleMessage cleanups from hell

Clean up libpurple/message.[ch] and remove the messages hash table as its current implementation will not fit our future needs

add a content-type property to PurpleMessage

Convert message timestamps from time_t to GDateTime and set default values

Make purple_message_new_outgoing take a parameter for the author.

Testing Done:
messages sent on bonjour, irc privmsg, irc chat, `/me` is broken i think unrelated.

Reviewed at https://reviews.imfreedom.org/r/124/