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/
if PLUGINS
if kwallet.found()
kwallet_moc = qt5.preprocess(
moc_headers: 'purplekwallet.h',
dependencies: qt5_dep,
)
kwallet_sources = [
'purplekwallet.cpp',
'purplekwallet.h',
kwallet_moc,
]
kwallet_plugin = library('purplekwallet', kwallet_sources,
dependencies : [kwallet, qt5_dep, libpurple_dep],
name_prefix : '',
install : true, install_dir : PURPLE_PLUGINDIR)
endif
endif