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/
---
id: im.pidgin.Pidgin3
runtime: org.gnome.Platform
runtime-version: '3.36'
sdk: org.gnome.Sdk
command: pidgin3
finish-args:
- --share=ipc
- --socket=fallback-x11
- --share=network
- --socket=pulseaudio
- --talk-name=org.freedesktop.Notifications
- --system-talk-name=org.freedesktop.Avahi
modules:
- packaging/flatpak/libpurple-common.yml
- packaging/flatpak/talkatu.yml
- name: pidgin3
buildsystem: meson
config-opts:
- -Dconsoleui=false
- -Ddoc=false
sources:
- type: dir
path: '.'