This moves the native show/hide calls to the generic code
for calculating viewable rather than in its own separate code
called from gdk_window_show/hide. This simplifies the code a bit,
but most significantly it means things are correctly shown when
they become viewable for other reasons than a show/hide call.

For instance, this fixes bug 590442 (gvim embedding) where the
toplevel GtkPlug is mapped by the embedder and we didn't previously
pick up that the native children became viewable and should be shown.