pidgin/pidgin

Use G_DECLARE_DERIVABLE_TYPE for PurpleConversation and additional cleanups

The setters for PurpleConversation are kind of crappy because of the
conversation cache in the purple_conversations api, which we'll address at a
later time.

Testing Done:
Compiled and ran locally, parted a chat and just signed out to verify no new issues.

Reviewed at https://reviews.imfreedom.org/r/613/
NOVELL_SOURCES = [
'nmfield.h',
'nmfield.c',
'nmconn.h',
'nmconn.c',
'nmconference.h',
'nmconference.c',
'nmcontact.h',
'nmcontact.c',
'nmevent.h',
'nmevent.c',
'nmmessage.h',
'nmmessage.c',
'nmrequest.h',
'nmrequest.c',
'nmrtf.h',
'nmrtf.c',
'nmuser.h',
'nmuser.c',
'nmuserrecord.h',
'nmuserrecord.c',
'novell.h',
'novell.c'
]
if DYNAMIC_NOVELL
novell_prpl = shared_library('novell', NOVELL_SOURCES,
dependencies : [libpurple_dep, glib, ws2_32],
install : true, install_dir : PURPLE_PLUGINDIR)
endif