bugfixes:

bugfixes:
1) ask for saving/mooing only when the user has choosen a file to load
2) opening a file doesn't call goto_top (removing all xml, even if the moo has
   been generated). This fix should enforce the invariant that a moo exists only
   if the corrsponding xml objects do.
3) the switch_page callback is inhibited before removing pages in
   sequentViewers#reset