Removed Qt dependencies for VegaChart example.

Now the ReadFile routine implementation is based on the standard
library only. Moreover the program accepts the "-b <base data url>"
as arguments.

Change-Id: I7baa981b72c6d68ab2fd5068f79722f3ab1387e6
Added new TestVegaRenderer test unit.

This new test compute the image error betwee chart image
generated by Vega Live Editor and images generated with
vtkVegaRenderer.

At present only test for arc, area and bar examples has been added.

Change-Id: I86a5e03d343a799aec8cd0921505ca25e027382d