script: Support layout manager properties

script: Support layout manager properties

Layout properties work similarly to child properties, with the added
headache that they require the 3-tuple:

  ( layout manager, container, actor )

to be valid in order to be inspected, parsed and applied. This means
using the newly added back-pointer from the container to the layout
manager and then rejigging a bit how the ScriptParser handles the
unresolved properties.

Similarly to the child properties, which use the "child::" prefix, the
layout manager properties use the "layout::" prefix and are defined with
the child of a container holding a layout manager.