docs: Clean up ClutterActor's long description
docs: Clean up ClutterActor's long description

The Actor's long description is a bit cluttered; it contains a section
on the actor's box semantics, on the transformation order and on the
event handling.

We should use <refsect2> tags to divide the Actor's description into
logically separated sections.

We should also add a section about the custom Scriptable properties that
ClutterActor defines, and the special handling of unit-based properties.

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