gis-assistant: prevent focus from getting stuck on a disabled button

Fixes #199 (closed)

Or appears to, I have no idea what I'm doing. An alternative could be to make sure that some other, enabled, widget is focused when loading the page. With my fix, I still have to press Tab once to get anywhere.

This feels like a Gtk bug to me. But anyway, maybe it helps.

EDIT: force-pushed a simpler version. I think the problem is that when update_navigation_buttons shows the Skip button, it hides but doesn't disable Next, so it's left enabled (sensitive) from the previous page or whatever. Now we just disable it before hiding.