actor: Fix child insertion issues
actor: Fix child insertion issues

The insert_child_at_index, insert_below and insert_above messed up the
first and last child pointers in various cases. This commit fixes all
the instances of first and last child pointers being stale or set to
NULL.

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