clean up "ref to Placement"

[ichthyo]

Development on Placements stalled a year ago, when there was just enough code nailed out to be able to continue in other parts of the Application. Especially the way of how to refer to a placement has been treated in an ad hoc manner and urgently needs cleanup.

• when to use a direct (language) ref to Placement<MObject>
• when to refer to one of the subtypes Placement<XYZ>
• what typedefs to use as a shorthand and where to define them. E.g. session::PMO
• where to prefer the new facility of a PlacementRef
• and where even to rely on a MObjectRef solely?

[ichthyo]

(In #117) taking responsibility for overviewing this task, while deferring it for later.

[ichthyo]

there is an interconnection to the query facilities on PlacementIndex: what form of "placement" (language ref, PlacementRef, equivalent Placement instance) is acceptable when invoking query operations?