- Add another parameter to IGUISkin::draw3DWindowBackground to allow getting the client area without actually drawing 
- Add function getClientRect to IGUIWindow for getting the draw-able area
- Documenation updates


git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@3073 dfc29bdd-3216-0410-991c-e03cc46cb475