Channel manager shows /webresources in list of pages

The channel manager shows /webresources in the list of pages. That should not happen.

Steps to reproduce:
1. login to the archetype CMS
2. open the only channel
3. click the 'Pages' button in the toolbar

Expected: /webresources is not listed as a page
Actual: /webresources is listed as a page. Clicking on it triggers an error popup saying "This page returned an error, going back." and shows the channel overview again.

I guess the HST should get a mechanism to exclude certain explicit sitemap items from being shown as a page in the channel manager.