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/
SILC_SOURCES = [
'buddy.c',
'chat.c',
'ft.c',
'ft.h',
'ops.c',
'pk.c',
'silc.c',
'silcpurple.h',
'util.c',
'wb.c',
'wb.h'
]
if DYNAMIC_SILC
silc_prpl = shared_library('silcpurple', SILC_SOURCES,
dependencies : [silc, libpurple_dep, glib],
install : true, install_dir : PURPLE_PLUGINDIR)
endif