Fixed issues with "hierarchy-changed" signal.

Fixed issues with "hierarchy-changed" signal.

GtkFileChooserDefault watches the toplevel and montitors "set-focus"
signal on it... however the connection needs to be remade when the
GtkFileChooserDialog is in an embedded toplevel.

Measure's taken: GtkWindow propagates hierarchy changes when
_gtk_window_set_is_toplevel() is called, gtk_widget_unparent()
unsets the widget's parent window earlier in the function so that
the possible hierarchy change is still able to properly access the hierarchy.

GtkFileChooserDefault checks if the "new" toplevel is indeed
gtk_widget_is_toplevel() but not the old one, GtkRange has been
updated to use gtk_widget_is_toplevel() inside it's hierarhcy_changed
vfunc, other classes already do this properly.