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.

tests/data/Makefile.am

@@ -13,6 +13,7 @@ json_files = \
 	test-animator-2.json			\
 	test-animator-3.json			\
 	test-state-1.json			\
+	test-script-layout-property.json	\
 	$(NULL)
 
 png_files = \

tests/data/test-script-layout-property.json

@@ -0,0 +1,16 @@
+[
+  { "id" : "manager", "type" : "ClutterBoxLayout" },
+
+  {
+    "id" : "container", "type" : "ClutterBox",
+    "layout-manager" : "manager",
+    "children" : [
+      {
+        "id" : "actor", "type" : "ClutterRectangle",
+        "layout::x-fill" : true,
+        "layout::y-align" : "center",
+        "layout::expand" : true
+      }
+    ]
+  }
+]