Cache results of panel_page_load_all()

panel_page_load_all() should indeed be cached as well.
On the one hand it is the only remaining function of this sort that we don't cache, making it an unintuitive exception.
On the other hand panel_page_load_all() does indeed get called twice if the page handler uses it, as menu will also call it. This is the case on admin/panels/panel-page.