menus: Avoid unnecessary right padding

menus: Avoid unnecessary right padding

We are using placeholders in the 'check' column
that are put in a size group, so that they all
take the same space once a check or radio is shown.

Unfortunately, for the inline-buttons option, we
were using a GtkBuiltinIcon as placeholder, and those
respect the -gtk-icon-size CSS property and take
a minimum size of 16px. Use a GtkGizmo instead to
get the expected result of no extra padding unless
there's a check or radio.

Fixes: #5839