vtkColorString - Helper class for defining a color through a string.

vtkColorString is an helper class for defining a color through one of
the following format:
- #RRGGBB               (6-digit hexadecimal number)
- #RRGGBBAA             (8-digit hexadecimal number)
- rgb(r, g, b)          (where r, g, b are in 0..255)
- rgba(r, g, b, a)      (where r, g, b, a are in 0..255)
- a CSS3 color name     (e.g. "steelblue")

Change-Id: Ied26e9fb8b2c3cbf6818ecf4c0504a2e3548de1b