pages: breadcrumbs handling: a new `breadcrumbs` list of dict is created in `template_data`. By default it is automatically filled by pages run to reach the requested URI, but a page can customize it. A breadcrumb data dict must have a `label`, should have an `url` and may have an `icon` (which is the name of a SàT Media well-known icon). Pages may now have a `label` attribute, which is used to automatically fill the crumb (otherwise page name then URI is used). A new `add_breadcrumb` method can be used to manually breadcrumb data, in which case auto-filling is disabled.