menusectionbox: Passthrough the focus function of custom widgets' gizmo

menusectionbox: Passthrough the focus function of custom widgets' gizmo

It broke keyboard focusing any widget added through the custom widget
menu feature. So for example if you put e.g. a custom check box widget
in a menu, you won't be able to focus it.

This is because the gizmo is mostly used to custom drawing with e.g.
CSS for small visual elements like scale markers. That's probably why
gizmo's default focus overrides block the focus from going through
the children. So this commit fixes it by overriding those and passing
the focus through the children.