VTK panel scalar array choice correction

[xavArtley]

Work in progress
Need to test if the correction handle correctly the colormap of the vtk mesh in all case and does not bring regression
Should correct in pyvista/pyvista#268 :

• RGBA color support inside a Jupyter Notebook pyvista/pyvista-support#15
• Make sure scalar/color mapping is working - sometimes the mapping is totally wrong. Blood vessels example shows this
• custom colormaps (see Ensure that events are not launched from the wrong thread #345) (except we must create a pyvista plotter since mesh.plot(use_panel=True) does not work anymore due to auto_close=True
  

Concerning :

• Send text/labels to VTKjs or at least don't throw an error (text added with add_point_labels and add_text)
  For me it does not throw error but the text does not show

@banesullivan it will be great if you could test it

[banesullivan]

These changes are great and much needed, thanks @xavArtley! I'll try to check this out and test when I have a minute.

Not to throw a curveball at you, but what if we reshifted focus to expanding itkwidgets and added support for itkwidgets in panel instead of using this VTKjs conversion script? itkwidgets is definitely the path forward for PyVista and it has a few open issues to make it more interactive and easier to treat like a PyVista plotter.

See my comment (pyvista/pyvista#364 (comment)) for some useful info/links to demos of itkwidgets

[philippjfr]

@xavArtley Ready to merge?

[xavArtley]

I think so I haven't encounter regressions