Updated backend for staticpage plugin:
1. Functionality remains mainly unchanged. Only preview display has been changed
from subpage-based access to id-based access, because subpage can be ambigous. The
new feature required changes to selected().
2. A hook-in is provided the generate a overview of all static pages when not editing
a static page. This makes the overview page more pleasent for users. The functionality
of the header remains unchanged. Only a list of pages is generated below the header.
The style of this list is taken from entry listing to create a consistent look and feel
for users.