Fix some issues we found in GPluginGtk in Pidgin

* 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.

Testing Done:
Compiled and debugged via a subproject in pidgin.

Reviewed at https://reviews.imfreedom.org/r/454/