In gedit_notebook_page_removed(), the page was already removed, so
calling gtk_notebook_get_current_page() there was wrong. It was maybe
due to a GTK+ change.

The list of focused pages contained only the previously focused pages,
not the current one. This commit changes that to also include the
currently focused page in the list, so in gedit_notebook_page_removed()
we know if the page removed was the current page or not.

Change also the order of the history, with the most recent focused page
at the first element of the list instead of at the end, so we don't need
to traverse the whole list each time.

Make also the code more robust.

https://bugzilla.gnome.org/show_bug.cgi?id=731784