introduce group item
this brings a group item in the style of genlist / gengrid group items.
The group item theme does currently not provide the signals in the theme
that would be needed for selecting it. This is kind of intended, but we
might need API to express that.
Reviewed-by: Cedric BAIL <email@example.com>
Differential Revision: https://phab.enlightenment.org/D9583