Added new property VisibilityForDesign to make control visible for desig...

...ning and developing.

I had a problem, where the control wasn't properly displayed in my app. It was quite hard to find the design problem. Therefore I added a property called VisibilityForDesign that allows developers to make the control visible in the designers (vs, blend).

My first attempt was to "override" the visiblity property, but that didn't work to I introduced a new Property, which binds to the Visibility Property of the UserControl it self. Default value is "collapsed". That way no changes in existing code needed.

In order to make the control visible, set VisibilityForDesign="Visible".