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 ENABLE_DOC
subdir('reference')
endif
if ENABLE_GTK
pidgin_man = configure_file(
input : 'pidgin3.1.in',
output : 'pidgin3.1',
configuration : man_conf,
install : true,
install_dir : get_option('mandir') / 'man1')
endif
if enable_consoleui
finch_man = configure_file(
input : 'finch3.1.in',
output : 'finch3.1',
configuration : man_conf,
install : true,
install_dir : get_option('mandir') / 'man1')
endif