**Add the ability to adding interactive 2D widgets from ParaView**

 - add the ability to adding interactive 2D widgets
 - add Equalizer filter with interactive 2d widget as example
 - add vtkFFT class that provides methods to perform Discrete Fourier Transforms

Here is an example of using vtkEqualizerFilter and vtkEqualizerContextItem.

Source data:
![Equalizer filter and Interactive 2D](interactive_2d_before.png  "Equalizer filter")

After applying the filter:
![Equalizer filter and Interactive 2D](interactive_2d_after.png  "Equalizer filter")

See the suggestions posted here:
[Discussion](https://discourse.paraview.org/t/is-there-any-abilities-to-add-a-interactive-widgets-on-linechartview-same-as-3d-widgets-on-renderview/2273)
