Make a `hide` and `show` method.

[RoarGronmo]

Instead of shrinking to 0 (which apparently doesn't hide the divider), could you please make an effort to make a hide and show method to apparently hide the containing widget (and its divider) or show the containing widget (and its divider) ?

The show parameter could be a part of the controller, in the same way as the size. You could even use it as a containing method like:

resizeableController.setSizes([
  ResizableSize.show(true),  //Visible, true could be default
  ResizableSize.show(false),  //Hidden 
]);

or the contrary

resizeableController.setSizes([
  ResizableSize.hide(false),  //Visible
  ResizableSize.hide(true),  //Hidden, true could be default
]);

Or more intuitive

resizableController.setVisible([
  true,
  false,
]);