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.
@garvinhicking: The code simply hooks into existing code without changing its functionality.
The intention was to add a more pleasent overview to staticpage while remaining its workflow.
The code for generating this list was taken from s9y core and was modified to the needs of 
staticpage. I asked for pulling this change, because it makes staticpage more attrative at first
view and makes navigation easier and more pleasent.