Add style to indicate development mode

This will help distinguish between a development mode version of WMT
versus a production mode without looking at the URL.
Add NullCommand inner class to ComponentSelectionMenu

It does nothing!
Asynchronously load ComponentCell menu

Load the list of available components from the server and display them
in the ComponentCell menu. As individual components from this list are
loaded into the client, update the ComponentCell menu. This is a step
in addressing #11.