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
On top of the existing "Settings" group in the settings.ini file we
should have two more groups:
Environment - contains all the configuration possible through
environment variables
Debug - contains all the possible debug variables
ClutterSettings should be able to load its initial state by using
configuration files in SYSCONFDIR and XDG_CONFIG_HOME. This allows
Clutter to have a system (and user) configuration on platforms that
do not have XSETTINGS bridges.
