Have CBillboardTextSceneNode use the default GUI font if no font is specified.  Also rename "shade_top"/"shade_down" to colorTop / colorBottom (and similarly for other "shade" parameters and members) in *BillboardSceneNode, *SceneManager and *TextSceneNode.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@1798 dfc29bdd-3216-0410-991c-e03cc46cb475