This adds new 'over' and 'under' transitions which work by moving
the new page over the previous one, or moving the previous page off
to reveal the new one. We also add an over/under combination that
is going to be used in GtkAboutDialog.
https://bugzilla.gnome.org/show_bug.cgi?id=707187
Add separate GtkStack and GtkStackSwitcher widgets that are an
alternative to GtkNotebook. Additionally, GtkStack supports
animated transitions when changing pages.
These widgets were initially developed in libgd.