make label attribute of Label widget const char* to avoid warnings
GraphicDisplay: don't reset background in drawImageRaw()
DynamicArray: implement find() and '>' and '<' operator
AbstractView: make attributes protected
* maintain a list of overlapping widgets (on top) and redraw when a lower
  widget has to be redrawn
* rename Widget::draw() to Widget::render() and call render from new
  Widget::draw() to have pre- and post-draw hooks for every widget