Clean up ClutterMainContext and clutter-private.h
Clean up ClutterMainContext and clutter-private.h

Do a better job at documenting the main context structure fields; remove
unused members; clean up the declarations.

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