[WPF] Fix SeparatorMenuItem Show / Hide

Hi.

I was porting a GUI from WinForms and at some point I had to add a hidden MenuItem and a SeparatorMenuItem to show them only in when a certain state is active.

On GTK+Windows it works fine, I just have to change the Visibility property or call Show or Hide methods.
But in WPF the separator cannot be hidden as I seems not to be a subclass of MenuItem.

Then I thought on remove temporary the Separator from the MenuItems list, saving the current position, and inserting it again at that position. The behavior almost the same.

I have not tested it on Mac, so I cannot say if the same issue occurs.