We used to explicitly map and unmap the child GtkFileChooserWidget when
mapping and unmapping the dialog, respectively.  Now that GtkWidget actually
unmaps child widgets (instead of avoiding that), we can assume that the
child GtkFileChooserWidget will be unmapped when we wanted it to be.

This fixes a warning that happened with the new GtkWidget invariant checker,
as we were mapping our child widget before calling our parent class' ::map() handler.

Signed-off-by: Federico Mena Quintero <federico@gnome.org>