GtkStack: add gtk_stack_get_child_by_name()

GtkStack: add gtk_stack_get_child_by_name()

This new method allows getting a widget from a GtkStack when we know its
name, and will also return NULL if there is no widget going by that
name.

Usage example would be to check if a child with a given name exists
before calling gtk_stack_set_visible_child_name().

https://bugzilla.gnome.org/show_bug.cgi?id=722588