Add _nm_setting_class_add_dbus_only_property(), for declaring
properties that appear in the D-Bus serialization, but which don't
correspond to GObject properties.

Since some property overrides will require examining settings other
than the setting that they are on (eg, the value of
802-11-wireless.security depends on whether an
NMSettingWirelessSecurity setting is present, and
NMSettingConnection:interface-name might sometimes be set from, eg,
bond.interface-name), we also update _nm_setting_to_dbus() to take the
full NMConnection as an argument, and _nm_setting_new_from_dbus() to
take the full connection hash.

Additionally, with some deprecated properties, we'll want to validate
them on construction, but we don't need to keep the value around after
that. So allow _nm_setting_new_from_dbus() to return a verification
error directly, so we don't need to store the value until the verify()
call.