Here is a patch that does two things (beside some cleanup):

* it adds 'getnodes()' methods that allow for more compact SQL queries.
* it adds a 'properties' argument to the 'filter()' methods, to allow to
fuse the query of properties with the original filter (again to reduce
the number of required SQL queries).

My plan is to adjust various functions (notably in cgi.templating) to
use these enhancements to reduce the number of queries made on the
database, and thus to speed up page rendering.

How does this look ?