updated doxyfile
UPDATE: version now 6.0-beta2
NEW: Widget::isDragging
FIX: layout refresh skipped if widget is in a free layout container
NEW: Widget::isDragging
CHANGE: display system setup is skipped if a display system has already been
specified

This is useful to let omegalib modules register a custom display system. Will
use this for DK2 oculus rift support.