Changes

disable "previous" button on welcome page
It is not clear from GTK docs, but it looks like switch-page cb would
never be called for an empty notebook. In addition, switch-change cb is
called from gtk_widget_show_all and it seems there
gtk_notebook_get_current_page always returns -1. The effect of all that
is that we should not check if notebook is empty in switch-page cb,
otherwise "previous" button will always be enabled on "welcome" page
when it is shown for the first time.
Simplify the code a bit while at it.
Closes https://github.com/maemo-leste/bugtracker/issues/168
Signed-off-by: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>

update debian/changelog
Signed-off-by: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>