Add scrolling to RhinoUIDockbar

Hi Dale,

thank you so much, I followed your example and implemented it into my code and it works wonderfully!

For documenting purposes, the way I “enabled” the scrollbars without controls:
I just have to call the following code whenever I expand the drawing, where displaySize is a CRect describing the height and width of the whole content (not only the visible one):

m_scroller->SetDisplaySize(displaySize);