A few of the settings plugins were calling nm_connection_clear_secrets()
from their finalize() method, but this call can emit signals, and by
the time finalize() runs, the object has a refcount of 0.  Signals
cannot be emitted from a finalized object, but instead could be
emitted from dispose() before the object is finalized.

Instead of moving the nm_connection_clear_secrets() to dispose() in each
plugin, make the behavior generic instead.  The settings plugins' parent
object is NMSettingsConnection, so clear secrets there.  Plus,
NMSettingsConnection caches system & agent secrets with NMSimpleConnection
objects, so clear secrets in NMSimpleConnection's dispose too.