Improve layout for Quilts.
 The algorithm for layering the nodes is now the one described in
 graphviz, and leads to a better layout (the leafs are not necessarily
 in the lowest layer)
This new algorithm is for now too slow for big genealogies, so will
 need to be sped up.
Part of #12
Optimize the computation of cut values in the layout algorithm
When we only display part of the quilts view, we only compute the
 rank ordering for part of the nodes, which speeds things up.
Fix import button use from the Quilts page
(stats): display total number of persons in the database
Fix sorting of events on the persona details page
Review style of the persona details page