removed SetFocus that was added to WM_MOUSEMOVE to handle keyboard focus

removed SetFocus that was added to WM_MOUSEMOVE to handle keyboard focus
with subwindows
Now using a solution similar to GLUT's, when receiving input, check if
mouse is actually on a child of the window passed in by the event, and
process as if event was on that window instead

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1348 7f0cb862-5218-0410-a997-914c9d46530a