shortcut-editor: show custom page while waiting for input

While waiting for keyboard input, as per the new proposed mockup,
the shortcut editor dialog should show a custom page with an icon
that indicates the required action. The current code, however, does
not expose this new customized page.

Fix that by adding this new page and controlling the time when it
shows and hides.

https://bugzilla.gnome.org/show_bug.cgi?id=769314