fix crash when closing a group chat with spellchk plugin enabled

Followed steps to reproduce from the bug report and neither experienced a crash or saw invalid reads related to this in valgrind.
Additionally tested that g_signal_handlers_disconnect_matched actually disconnects signal handlers when unloading the spellchk plugin.
Also tested following test case:
* connect an IRC account
* enable spellchk plugin
* join a chat
* disable spellchk plugin
* close chat window
which led to an invalid read before I added the g_object_remove_weak_pointer call. Now, I don't see any invalid reads related to this in valgrind anymore.