The Selection visualization in EditVierw is organized
to use XOR for visualization, thus DrawSelection is
used e.g. before and after changes to the Selection.
Ensured athat all changers of selection have to use
SetEditSelection by making return of GetEditSelection
const. This allows to use a central and safe
SelectionChanged which is needed for alternatives of
Selection visualization.

Change-Id: I994553f3be6b58fd595aa500922d8d1d8ddd36d4