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/
/*
* finch
*
* Finch is the legal property of its developers, whose names are too numerous
* to list here. Please refer to the COPYRIGHT file distributed with this
* source distribution.
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, see <https://www.gnu.org/licenses/>.
*/
#ifndef FINCH_H
#define FINCH_H
#ifndef __GI_SCANNER__ /* hide this bit from g-ir-scanner */
# ifdef FINCH_COMPILATION
# error "finch source files should not be including finch.h"
# endif /* FINCH_COMPILATION */
#endif /* __GI_SCANNER__ */
#ifndef FINCH_GLOBAL_HEADER_INSIDE
# define FINCH_GLOBAL_HEADER_INSIDE
#endif /* FINCH_GLOBAL_HEADER_INSIDE */
@FINCH_H_INCLUDES@
#undef FINCH_GLOBAL_HEADER_INSIDE
#endif /* FINCH_H */