Replace vtkActor with vtkViewProp in RenderedData interface, don't assume a single main view prop

- Allow to use VTK's context views (2D)
- mainActor() doesn't always make sense (e.g., RenderedVectorGrid3D)
move RenderedData hierarchy to core/rendered_data/
fix regression: apply renderProperty() in ImageProfilePlot and RenderedImageData