Start to eliminate use of 'index' for PropertyTreeNodes.

This is a design flaw. 'index' is _not_ a property of the PTN. It's a
property of the list that holds the PTN. A PTN could be part of
multiple lists at the same time. This is only the first step of fixing
this.
Merge branch 'master' of github.com:taskjuggler/TaskJuggler