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.