The latest concept for a crosshair for the mxnmultiwidget is the CrosshairManager which provides three orthogonal plane geometries (as pane nodes) for each individual render window.
We should define a new concept for render window crosshairs, probably a dedicated data type and a dedicated mapper class, that can also be used for the StdMultiWidget.

Problems with the current approach:

• the plane nodes / plane geometries are dependent on the renderer geometry which seems to be unnecessary
  • we had a point set for crosshair before, which was only dependent on the world position but that had its own flaws (see {T26627}).
  • inspecting the plane geometry data mapper (PlaneGeometryDataMapper2D::CreateVtkCrosshair) could give us more insights

• plane nodes clutter the data manager