clutter-settings: Mark window-scaling-factor as fixed when set by the app

When an application sets the scaling factor manually we should mark it as fixed
and not override it when the xsettings change. This matches GDKs behaviour.

In order for this to work we cannot use the same path when setting the value
internally so introduce a _clutter_settings_set_property_internal and use it
for that.

https://bugzilla.gnome.org/show_bug.cgi?id=735244