[API] provider support for dynamic composition is optional

All views are expected to implement dynamic composition handling
by listening for the "add" and "remove" events and then calling
"collection.load()" when they are ready to handle these events.

However, it does not make sense that every composition provider will
be dynamic, so implementing support for dynamic composition should
not be a requirement.  This commit removes that requirement.

Fixes #1914

src/api/composition/CompositionCollection.js

@@ -76,20 +76,22 @@ define([
             throw new Error('Event not supported by composition: ' + event);
         }
 
-        if (event === 'add') {
-            this.provider.on(
-                this.domainObject,
-                'add',
-                this.onProviderAdd,
-                this
-            );
-        } if (event === 'remove') {
-            this.provider.on(
-                this.domainObject,
-                'remove',
-                this.onProviderRemove,
-                this
-            );
+        if (this.provider.on && this.provider.off) {
+            if (event === 'add') {
+                this.provider.on(
+                    this.domainObject,
+                    'add',
+                    this.onProviderAdd,
+                    this
+                );
+            } if (event === 'remove') {
+                this.provider.on(
+                    this.domainObject,
+                    'remove',
+                    this.onProviderRemove,
+                    this
+                );
+            }
         }
 
         this.listeners[event].push({
@@ -124,20 +126,22 @@ define([
         if (this.listeners[event].length === 0) {
             // Remove provider listener if this is the last callback to
             // be removed.
-            if (event === 'add') {
-                this.provider.off(
-                    this.domainObject,
-                    'add',
-                    this.onProviderAdd,
-                    this
-                );
-            } else if (event === 'remove') {
-                this.provider.off(
-                    this.domainObject,
-                    'remove',
-                    this.onProviderRemove,
-                    this
-                );
+            if (this.provider.off && this.provider.on) {
+                if (event === 'add') {
+                    this.provider.off(
+                        this.domainObject,
+                        'add',
+                        this.onProviderAdd,
+                        this
+                    );
+                } else if (event === 'remove') {
+                    this.provider.off(
+                        this.domainObject,
+                        'remove',
+                        this.onProviderRemove,
+                        this
+                    );
+                }
             }
         }
     };