Improve the GUI concept

[bknecht, rgrieder]

Outline

Orxonox needs a concept for the menu. This concerns all kinds of ways how we might want to use elements from CEGUI.

Current Status

We use CEGUI to display a GUI. You can either use version 0.6 or 0.5 and you don't have to worry about the scripting plugin, we've dealt with that internally.

At the moment, the menu doesn't do much: A camera as well as an empty SceneManager are created (Ogre can't render without). There is also a configured interface to the lua scripting engine.

To be done

Clean up some code and add new functions that are needed. This mostly concerns C++ code. The actual GUI code (in lua) is found in this ticket.
And we especially need the input system to be able to use non exclusive input mode if required. That means the mouse cursor can move over a non full screen window without being bounded to the orxonox window.
There's also some work to be done with the GameStates, but that's in another ticket.

[rgrieder]

A basic GUI implementation has been done with lua scripting interface.

[rgrieder]

Updated the text and merged with ticket #251