back-button: Update on AdwNavigationPage::showing instead of ::pushed
When showing the page, we try to focus a widget in it before the back button is visible. If there are no other focusable widgets, we fail, and that's not good.
Make sure to show the (initially hidden) back button when showing the page, before focusing anything.
Meanwhile we don't need the pushed signal anymore. That's what it was originally for, but it's emitted too late to be useful here.