Reset position in FactTree only when needed

Created by: rhertzog

Right now FactTree.set_facts() is always resetting the position of the view, bringing the user back to the top but this function is called regularly to update with the latest data even when the user has not changed anything in the selection widget... which is really annoying when you are reviewing the list of facts.

Let's distinguish between both cases with a "reset=True" attribute that we use when we generate an entirely new set of facts to be displayed.

This makes #623 obsolete. Thanks to @mwilck for the initial analysis.

Fixes #594 (closed)