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/
[Desktop Entry]
Name=Pidgin
GenericName=Internet Messenger
Comment=Chat over IM. Supports Jabber/XMPP, and more
Exec=purple3-url-handler %u
TryExec=purple3-url-handler
Icon=pidgin
StartupNotify=false
Terminal=false
NoDisplay=true
Type=Application
Categories=Network;InstantMessaging;RemoteAccess;ConsoleOnly;
MimeType=x-scheme-handler/gg;x-scheme-handler/irc;x-scheme-handler/sip;x-scheme-handler/xmpp;
@USES_MM_CHAT_SECTION@