The latest concept for a crosshair for the mxnmultiwidget is the `CrosshairManagaer` 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
- there is no concept of sharing crosshair position between individual render windows
- will be useful if render windows show different data / have different camera views on an image but the clicked / focused position of render window A should be visible in render window B
- could simplify {T29393}
- plane nodes clutter the data manager
- dedicated data types could also provide more information, like pixel information / value