Style visibility=HIDDEN has no effect on GTK

### Describe the bug

Using this code:
```
self.sel_control = toga.Selection(
items=[], style=tp.Pack(flex=1), accessor="name",
on_change=self.on_selection_control_sel_changed)
box.add(self.sel_control)
self.sel_control.style.update(visibility=toga.style.pack.HIDDEN)
```
The widget is still visible. Makes it impossible to have a box with different widgets showing only the one matching an input type. And no the OptionContainer is not an option since it has tabs and does not guarantee invisibility.

### Steps to reproduce

Use the code above. Widget is still visible

### Expected behavior

Widget is invisible

### Screenshots

_No response_

### Environment

Linux, most recent Toga

### Logs

_No response_

### Additional context

_No response_

Start by reproducing the provided Selection example on Linux with the GTK backend, then trace how the visibility style is applied to the widget. Done means setting visibility to HIDDEN makes the widget invisible while preserving the expected behavior of other visibility values.

Written by the indexing model from the issue text.