actor: Add a custom scriptable "margin" property

The property uses an array with the following CSS style syntax

 [ top, right, bottom, left ] or
 [ top, left/right, bottom ] or
 [ top/bottom, left/right ] or
 [ top/right/bottom/left ]

https://bugzilla.gnome.org/show_bug.cgi?id=676367

tests/data/Makefile.am

@@ -16,6 +16,7 @@ json_files = \
 	test-animator-3.json			\
 	test-state-1.json			\
 	test-script-timeline-markers.json	\
+	test-script-margin.json 		\
 	$(NULL)
 
 png_files = \

tests/data/test-script-margin.json

@@ -0,0 +1,22 @@
+[
+  {
+    "id" : "actor-1",
+    "type" : "ClutterActor",
+    "margin" : [ 10 ]
+  },
+  {
+    "id" : "actor-2",
+    "type" : "ClutterActor",
+    "margin" : [ 10, 20 ]
+  },
+  {
+    "id" : "actor-3",
+    "type" : "ClutterActor",
+    "margin" : [ 10, 20, 30 ]
+  },
+  {
+    "id" : "actor-4",
+    "type" : "ClutterActor",
+    "margin" : [ 10, 20, 30, 40]
+  }
+]