Improve menu.

The grey bar was not very intuitive to use, and it does not work with
all browsers. While elinks ignores the css and renders the menu at
the top of the page, at least one popular cell phone browser neither
shows the bar nor the menu, making it impossible to navigate the pages.

This commit reworks the css and html code to always show an hamburger
icon at the top left corner of the page. Moving the pointer on the
icon opens the menu.

Suggested-by: Sanja Jasek <S.Jasek@exeter.ac.uk>
Tested-by: Maša Lenuzzi <masa.lenuzzi@tuebingen.mpg.de>