text: Add missing introspection annotations
text: Add missing introspection annotations

Add allow-none annotations where appropriate

heysion/clutter-clone,djdeath/clutter-android,kerrickstaley/clutter-vala,Distrotech/clutter,collects/clutter,djdeath/clutter,jigpu/clutter,djdeath/clutter,GNOME/clutter,GNOME/clutter,Distrotech/clutter,jigpu/clutter,Distrotech/clutter,djdeath/clutter-android,heysion/clutter-clone,kerrickstaley/clutter-vala,collects/clutter,collects/clutter,Distrotech/clutter,jigpu/clutter,heysion/clutter-clone,djdeath/clutter,jigpu/clutter,djdeath/clutter-android,kerrickstaley/clutter-vala,ebassi/clutter,jigpu/clutter,GNOME/clutter,heysion/clutter-clone,djdeath/clutter-android,kerrickstaley/clutter-vala,ebassi/clutter,GNOME/clutter,collects/clutter,ebassi/clutter,djdeath/clutter,djdeath/clutter,ebassi/clutter,djdeath/clutter-android,heysion/clutter-clone,Distrotech/clutter,Distrotech/clutter,ebassi/clutter,jigpu/clutter,GNOME/clutter,collects/clutter,djdeath/clutter