Modified tab panel

Added some grey rects which could be the different modules. Can click
the tab panel to close it.
Remove magic number

All buttons should appear on iOS too
Add getModuleIcon method

The method should return an Actor to represent the icon to be displayed
by the module switcher.

Also make changes to the touchDown method for when being used in-game.
GameScreen keeps a list of Modules

This way it can instantiate the list of modules for a particular
mission as needed.

When you click on an icon, you get taken to that module.