So we can style it differently than :hover.
We already have a active state for the menu items which includes
more than hover. For example, when the keyboard focus moves to a item
or we select programatically a item.
For this reason we need a style class named active for the meaning we
give to it in menu items, and a pseudo class active with the meaning
CSS has.
https://bugzilla.gnome.org/show_bug.cgi?id=744680
8d66fff2aa