Checkboxradio: Use the state instead of the icon class in the selector.

There are no checkbox or radio on and off icons. There is only a check
icon. So it makes more sense if we use the class that indicates the
state in selectors.
Selectmenu: Multiple custom menu items should get the checkbox state class.
Theme: Simplified selectors for checkbox and radio style.
Icons: Simplified selector for checkbox icon.
Checkboxradio: Don't add icon classes because we don't use them anymore.
Tests: Updated form integration test.