Splitted up Widgets' Draw methods into Draw() and DrawInternally
Draw Notebooks' background depending on a given dirty rectangle, not theoretical widget dimensions
Let label have at maximum dirtyRect's width when drawing, not ignoring this restriction anymore
[Widget] When Backgroundcolor has changed, only Invalidate if value actually varies from the previous value
[Widget] Introduced ViewportProxy-offset for offsetting Invalidate()-requests properly if widget is only halfway visible due to scrolling etc.