Remove GtkLayout

Remove GtkLayout

The need of a specialised fixed layout container that can be placed into
a GtkScrolledWindow ceased to exist once GtkScrolledWindow gained the
ability to automatically interpose a GtkViewport when adding a child
that does not implement GtkScrollable.

All the other justifications that led to the existence of GtkLayout as a
separate widget from GtkFixed have been largely made irrelevant in the
20 years since its inception.