Implement NativeMenuItemSeparator.IsVisible

[setoy]

Is your feature request related to a problem? Please describe.

Changing the visibility of the NativeMenuItemSeparator is useful in cases where you only want to show a separator on a certain condition.

Describe the solution you'd like

Implement IsVisible on NativeMenuItemSeparator similar to IsVisible on NativeMenuItem (like done here: #7952).

Describe alternatives you've considered

No response

Additional context

I want to show a separated "Logout" button in the TrayIcon - but only show the menu item (including its separator) when a user is actually logged into my application.