Dropdown menu fixes and adjustments

Suggested fix for issue #275, and a few little tweaks. Fixed the display gaps, but also switched to hiding submenus using clip() rather than display:none for improved accessibility. I converted the dropdown width to a variable for my purposes; I figured it would make the styling math more obvious for folks who want to adjust it. I allowed for up to 4 dropdown levels, which is likely overkill. My compiled CSS is not minified (for better or worse).