Wrapper for actors inside a layout manager
<clutter-layout-meta> is a wrapper object created by
<clutter-layout-manager> implementations in order to store
child-specific data and properties.
A <clutter-layout-meta> wraps a <clutter-actor> inside a
<clutter-container> using a <clutter-layout-manager>.
<clutter-layout-meta> is available since Clutter 1.2