Fix display behavior when nav pane is resized to a very short height


When the NavigationPane is resized to a height that is less than the height of all buttons combined, it displays...oddly. The easiest way to see this is to put a control on a form, add about 4 pages, and then run the form and shrink it vertically. Then do the same with Outlook 2007 and see how it behaves.
A MinimumHeight property should probably be added to the NavigationPanePage class, or a MinimumPageHeight property to the NavigationPane class if you don't want to allow this to be set at the page level.