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.
The fact that NMRemoteConnection has to be an NMConnection and
therefore can't be an NMObject means that it needs to reimplement bits
of NMObject functionality (and likewise NMObject needs some special
magic to deal with it). Likewise, we will need a daemon-side
equivalent of NMObject as part of the gdbus port, and we would want
NMSettingsConnection to be able to inherit from this as well.
Solve this problem by making NMConnection into an interface, and
having NMRemoteConnection and NMSettingsConnection implement it. (We
use some hacks to keep the GHashTable of NMSettings objects inside
nm-connection.c rather than having to be implemented by the
implementations.)
Since NMConnection is no longer an instantiable type, this adds
NMSimpleConnection to replace the various non-D-Bus-based uses of
NMConnection throughout the code. nm_connection_new() becomes
nm_simple_connection_new(), nm_connection_new_from_hash() becomes
nm_simple_connection_new_from_hash(), and nm_connection_duplicate()
becomes nm_simple_connection_new_clone().
