settings: Handle window scaling factor internally
settings: Handle window scaling factor internally

We want the settings object to handle setting and getting the
window scaling factor value, both through backend-specific settings and
through the CLUTTER_SCALE environment variable. This means turning the
ClutterSettings:window-scaling-factor property into a readwrite one,
instead of write-only, so that ClutterStage implementations will be able
to query the window scaling factor on construction.

https://bugzilla.gnome.org/show_bug.cgi?id=705915

djdeath/clutter,GNOME/clutter,jigpu/clutter,ebassi/clutter,jigpu/clutter,Distrotech/clutter,Distrotech/clutter,GNOME/clutter,Distrotech/clutter,ebassi/clutter,djdeath/clutter,ebassi/clutter,jigpu/clutter,djdeath/clutter,djdeath/clutter,jigpu/clutter,Distrotech/clutter,jigpu/clutter,djdeath/clutter,jigpu/clutter,ebassi/clutter,GNOME/clutter,GNOME/clutter,djdeath/clutter,Distrotech/clutter,ebassi/clutter,GNOME/clutter,Distrotech/clutter