Ensure that checkboxes are shown in menus

The old code tried to add an icon for all menu items, even for
empty icon names. Most themes (including the default ‘Fusion’ theme,
but not KDE’s ‘Breeze’) don’t display checkboxes when a menu item
already has an icon. The result was that unchecked checkboxes
were invisible and checked checkboxes looked strange (like large,
empty rectangles).

With this commit, icons are only added for actions that actually
*have* icons, and the checkboxes are thus correctly displayed no matter
what theme the user has.