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

This fixes a warning from gtk_widget_verify_invariants(), as we were mapping
our child widget before calling our parent class' ::map() handler. Bug #659257.