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 (closed)