Allow dropdown to act as a normal dropdown

Adding the .select class to .menu makes the dropdown act like a select.
Otherwise, it acts like a simple dropdown, keeping the "dropdown-button" value when clicking on a component of the dropdown.
It can be useful if you need a simple dropdown, but still use the version where it acts like a select.
Well, I looked at the other JS file from refills, and used single quotes based on the use in other files. Most files use single quotes, so I went with single quotes for consistency.