initialisation: ScopePath, QueryFocusStack, bottom placement

[ichthyo]

care for proper initialisation of the subsystems related to PlacementIndex. There are some static variables and constants -- it seems better to do a dedicated initialisation when the session comes up first, instead of relying on static initialisation.

[ichthyo]

sidenote: PlacementRef now checks if the session interface is up, before doing any access...

[ichthyo]

checked and modified.
By passing in the root element, and by changing all iterative processing within the index implementation into iterator based processing, I got rid of all globals. The "invalid scope path" was changed such as not to pull up the session anymore, thus it can remain statically initialised.