Fix a showstopper where menu item is not clickable

This pull request fixes the [showstopper issue MAGN-733](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-733) where menu item is not clickable. The problem was that the ```Border``` XAML element was too narrow, and hidden when the menu item is unchecked. The fix is: instead of hiding the ```Border``` element, make it always visible and make its ```Background``` attribute to be ```Transparent```. What this essentially mean to WPF is that the visual element is hit-testable, thereby achieving what we wanted for our menu item.