DEV: Introduce <DPageHeader /> and <DPageSubheader /> components (#30146)

This converts the `<AdminPageHeader />` component and the
`<AdminPageSubheader />` components into new components
that can be used outside of admin, and updates the CSS classes.
Also introduces a `<DPageActionButton />` component and child
components for the header action buttons.

I have to keep the old admin-only components around for
now until plugins are updated, then we can remove it,
and remove the re-exports that are done within
admin-page-action-button.gjs

spec/system/page_objects/components/admin_header.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+module PageObjects
+  module Components
+    # TODO (martin) Delete this after plugins have been updated to use DPageHeader
+    class AdminHeader < PageObjects::Pages::Base
+      def has_tabs?(names)
+        expect(page.all(".d-nav-submenu__tabs a").map(&:text)).to eq(names)
+      end
+
+      def visible?
+        has_css?(".d-page-header")
+      end
+
+      def hidden?
+        has_no_css?(".d-page-header")
+      end
+    end
+  end
+end