Disable some pages in an `AdwViewStack`
For many applications, not all pages are usable in every state. So they need a way to prevent users from going to those pages. Developers can, of course, only add the pages to the view stack when they become available, but to users, the experience of certain pages magically appearing in the switcher is not a good one, if they discover the newly added pages at all.
So I propose to add some mechanism to disable a page so that the button in the switch is grayed out (disabled).