Make enum- and flags-valued properties use GParamSpecEnum and
GParamSpecFlags, for better introspectability/bindability.

This requires no changes outside libnm-core/libnm since the expected
data size is still the same with g_object_get()/g_object_set(), and
GLib will internally convert between int/uint and enum/flags GValues
when using g_object_get_property()/g_object_set_property().