metrics: Turn breadcrumb into a PageSection
metrics: Turn breadcrumb into a PageSection

That way it has exactly the same height as the header on the Overview
(or kdump, etc.) page, and thus there is less jumping. This also allows
us to add more items to the right of the header bar, which is not
possible with `<Page>`'s groups (as they are aligned vertically).

martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,garrett/cockpit,mvollmer/cockpit,garrett/cockpit,martinpitt/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,mvollmer/cockpit,garrett/cockpit,garrett/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,garrett/cockpit,cockpit-project/cockpit