Create a no operation credential provider and fix a number of other issues

* Create a "None" credential provider that allows users to disable credential
storing and only allows them to enter passwords manually.
* Use g_signal_connect_object in PidginCredentialProviderStore.
* Move purple_credential_manager_shutdown after purple_plugins_uninit.
* Set the active credential provider to NULL during shutdown.

Testing Done:
Compiled and ran locally verified that we no long hang on exit if we've opened the preferences window.

Reviewed at https://reviews.imfreedom.org/r/456/