Avoid critical errors when creating multiple application menus

First, create a composer action corresponding to the app menu, instead
of reusing the one from the controller.  Second, create the menu from a
MenuModel, instead of from a UIManager, so we can have many of them.

https://bugzilla.gnome.org/show_bug.cgi?id=746504

ui/composer.glade

@@ -236,6 +236,13 @@
         <property name="icon_name">edit-copy-symbolic</property>
       </object>
     </child>
+    <child>
+      <object class="GtkAction" id="gear menu">
+        <property name="label" translatable="yes">Application Menu</property>
+        <property name="icon_name">emblem-system-symbolic</property>
+      </object>
+      <accelerator key="F10"/>
+    </child>
   </object>
   <object class="GtkBox" id="composer">
     <property name="visible">True</property>