libnm: always set from_dbus_fcn() property hook

When looking at a property, it should always be clear how it is handled.
Also the "default" action should be an explicit hook.

Add _nm_setting_property_from_dbus_fcn_gprop() and set that as
from_dbus_fcn() callback to handle the "default" case which us
build around g_object_set_property().

While this adds lines of code, I think it makes the code easier to
understand. Basically, to convert a GVariant to a property, now all
properties call their from_dbus_fcn() handler, there is no special casing.
And the gprop-hook is only called for properties that are using
_nm_setting_property_from_dbus_fcn_gprop(). So, you can reason about
these two functions at separate layers.

src/libnm-core-impl/nm-setting-user.c

@@ -577,10 +577,12 @@ nm_setting_user_class_init(NMSettingUserClass *klass)
         properties_override,
         obj_properties[PROP_DATA],
         NM_SETT_INFO_PROPERT_TYPE_GPROP(NM_G_VARIANT_TYPE("a{ss}"),
-                                        .gprop_from_dbus_fcn = _nm_utils_strdict_from_dbus,
+                                        .typdata_from_dbus.gprop_fcn = _nm_utils_strdict_from_dbus,
                                         .typdata_to_dbus.gprop_type =
                                             NM_SETTING_PROPERTY_TO_DBUS_FCN_GPROP_TYPE_STRDICT,
-                                        .compare_fcn = compare_fcn_data));
+                                        .compare_fcn   = compare_fcn_data,
+                                        .from_dbus_fcn = _nm_setting_property_from_dbus_fcn_gprop,
+                                        .from_dbus_is_full = TRUE));
 
     g_object_class_install_properties(object_class, _PROPERTY_ENUMS_LAST, obj_properties);