Toggle 'open' class on .dropdown-submenu el

This allows persistent 'active/open' state styling. Currently, when the cursor is on a `dropdown-submenu` element, it can be styled using the `:hover` state, but when the cursor drops to the open 3rd tier menu, that state reverts and it appears 'inactive'. This change allows you to use a `.dropdown-submenu.open > a` CSS declaration and have more control.