stage/manager: Set :default-stage as read-only
stage/manager: Set :default-stage as read-only

The StageManager:default-stage property should be read-only, since
the default stage is owned by Clutter itself.

collects/clutter,ebassi/clutter,djdeath/clutter,djdeath/clutter-multithreaded,kerrickstaley/clutter-vala,djdeath/clutter-android,collects/clutter,heysion/clutter-clone,heysion/clutter-clone,djdeath/clutter-multithreaded,heysion/clutter-clone,spatulasnout/clutter,GNOME/clutter,heysion/clutter-clone,jigpu/clutter,jigpu/clutter,Distrotech/clutter,Distrotech/clutter,jigpu/clutter,spatulasnout/clutter,djdeath/clutter,rkudiyarov/clutter_osx,GNOME/clutter,rkudiyarov/clutter_osx,Distrotech/clutter,ebassi/clutter,ebassi/clutter,Distrotech/clutter,rkudiyarov/clutter_osx,djdeath/clutter,dlespiau/clutter,Distrotech/clutter,djdeath/clutter-android,collects/clutter,djdeath/clutter,collects/clutter,Distrotech/clutter,djdeath/clutter-android,kerrickstaley/clutter-vala,kerrickstaley/clutter-vala,djdeath/clutter-android,kerrickstaley/clutter-vala,spatulasnout/clutter,spatulasnout/clutter,GNOME/clutter,nobled/clutter,jigpu/clutter,dlespiau/clutter,djdeath/clutter,spatulasnout/clutter,jigpu/clutter,djdeath/clutter-multithreaded,ebassi/clutter,djdeath/clutter,djdeath/clutter-android,rkudiyarov/clutter_osx,djdeath/clutter-multithreaded,nobled/clutter,GNOME/clutter,GNOME/clutter,dlespiau/clutter,nobled/clutter,ebassi/clutter,collects/clutter,heysion/clutter-clone,jigpu/clutter