Improving the selection mecanism

This PR improves the overall selection mecanism. It implements things that were done in a branch of the original fibernavigator more than a year ago, but were never merged.

It mainly adds the SelectionTree: a class that manages the hierarchy of selection objects, and their relations. This class is not responsible of managing all the selection of fibers, the computation of stats, etc.

It also fixes issues #87 and #88.