Fix some issues we found in GPluginGtk in Pidgin
Review Request #454 — Created Jan. 26, 2021 and submitted
* GPluginGtkInfo needs to make sure it has a plugin before trying to work with it. * GPluginGtkStore needs to use g_signal_connect_object as it's passing itself as a data parameter to signals from GPluginManager.
Compiled and debugged via a subproject in pidgin.