Added navigation through the GUI using tab and the shift and control keys. Added keyboard navigation for most elements. Fixed that annoying bug where you had to click twice to change focus.
Added IGUIElement::isMyChild, isPointInside,  SEvent.GUIEvent.Element. 
Changed setFocus and removeFocus to return bools.
changed quaternion::toAngleAxis to use iszero. 

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@763 dfc29bdd-3216-0410-991c-e03cc46cb475