actor: Default to a fixed layout manager
actor: Default to a fixed layout manager

In case no layout manager was set during construction, we fall back to a
FixedLayout. The FixedLayout has the property of making the fixed
positioning and sizing API, as well as the various Constraints, work
out of the box.

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