pidgin/pidgin

Remove ui_data from PurpleConversation and move finch and pidgin to g_object_[gs]et_data.

Testing Done:
Compiled and ran pidgin, but didn't run finch.

Reviewed at https://reviews.imfreedom.org/r/467/
For information on translating Pidgin, libpurple, and Finch, please see
our wiki page at https://developer.pidgin.im/wiki/TipsForTranslators