Rework MeshViewer:

- Break up EventHandler into more functions
- Add enums for all ID's
- Fix scaling display
- Add quick-scale buttons for *10 and * 0.01


git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@2953 dfc29bdd-3216-0410-991c-e03cc46cb475