The child property is watched by the StackSwicther which in turns sets a
needs-attention css class on the corresponding button, so that the theme
can for instance show a throbbing animation if one of the hidden pages
needs the user attention.
https://bugzilla.gnome.org/show_bug.cgi?id=707153
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.