Remove all the old 2.x and 3.x version annotations.
GTK+ 4 is a new start, and from the perspective of a
GTK+ 4 developer all these APIs have been around since
the beginning.
The GtkStack and GtkStackSwitcher code did not really
follow GTK+ conventions for includes. Fix that, and also
fix up a case of gpointer vs gpointer* confusion
in gtkstack.c.
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.