Composition policy takes child instance

The composition policy now takes a child instance instead
of the child type, as in all cases we have access to the child
object.

This allows new-style objects to be contained by old-style objects.

Updated all composition policies to use standardized argument names
instead of `context` and `candidate`; this makes it easier to
understand.

Updated AddActionProvider to hardcode the object types supported.

platform/containment/src/ComposeActionPolicy.js

@@ -45,9 +45,7 @@ define(
         ComposeActionPolicy.prototype.allowComposition = function (containerObject, selectedObject) {
             // Get the object types involved in the compose action
             var containerType = containerObject &&
-                    containerObject.getCapability('type'),
-                selectedType = selectedObject &&
-                    selectedObject.getCapability('type');
+                containerObject.getCapability('type');
 
             // Get a reference to the policy service if needed...
             this.policyService = this.policyService || this.getPolicyService();
@@ -57,7 +55,7 @@ define(
                 this.policyService.allow(
                     'composition',
                     containerType,
-                    selectedType
+                    selectedObject
                 );
         };