[API] Add containment methods

src/api/composition/CompositionCollection.js

@@ -94,6 +94,10 @@ define([
         }
     };
 
+    CompositionCollection.prototype.canContain = function (domainObject) {
+        return this.provider.canContain(this.domainObject, domainObject);
+    };
+
     CompositionCollection.prototype.destroy = function () {
         if (this.provider.off) {
             this.provider.off(

src/api/composition/DefaultCompositionProvider.js

@@ -59,6 +59,10 @@ define([
         );
     };
 
+    DefaultCompositionProvider.prototype.canContain = function (domainObject, child) {
+        return true;
+    };
+
     DefaultCompositionProvider.prototype.remove = function (domainObject, child) {
         // TODO: this needs to be synchronized via mutation
         var index = domainObject.composition.indexOf(child);