Adding a "PopupMenuRadioButtonItem" wouldn't work well, because we'll
need radio-button indicators on multiple different styles of menu
item. Also, the current design draws the indicator in the menu item's
padding, so it's sort of special anyway. So just add support at the
BaseMenuItem level.
Also, redo the menu/menuitem padding so that all the horizontal
padding is in the menu item, or else the indicator dot will show up in
the wrong spot.
https://bugzilla.gnome.org/show_bug.cgi?id=631193
9d94da83d7