KMenu should hide when child content is empty

[claustres]

For instance if a KMenu contains a component with an empty panel it will still be visible.

[cnouguier]

That means the KMenu is not really empty as it contains a component.

[cnouguier]

As the content is a prop and it is forwarded to the KPanel, we can check whether the content is empty or not. But it is the case already.

[cnouguier]

After investigation, there is finally content which is then hidden via the visibility mechanism on the actions