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
Merged vtkColorString into vtkNamedColors.

Removed support for #RRGGBBAA format since is not a valid HTML
color string format. See CSS3 color reference documentation.

Added support for the #RGB short format.
Added support for percentage values in rgb and rgba functional
formats.

vtkTestClorString.cxx source code has been merged into
vtkTestNamedColors.cxx.

Added a new method for get an HTML color string in hexadecimal
format from an instance of vtkColor3ub.

Added a new method for setting a color by name:
void SetColor(const vtkStdString & name,
                     const vtkStdString & htmlString);

No color is set if the name is empty or if `htmlString` is not a valid
HTML color string.

Change-Id: Ib9b9d2fa96a4dd5ac476b7bed2349cf4462ea9a9
I added a RGBAToHTMLColor and made some minor changes.

Added a vtkStdString RGBAToHTMLColor(const vtkColor4ub & rgba)
method  which given a vtkColor4ub instance as input color returns
a valid HTML color string in the `rgba(r, g, b, a)` format.

Change-Id: I490d173c2fc2d66869cda4d0db7ae6f632607818
Merge topic 'feature/color-string' into master

1f2cdbfc I added a RGBAToHTMLColor and made some minor changes.
04c2b8ce Merged vtkColorString into vtkNamedColors.
126ce11b vtkColorString - Helper class for defining a color through a string.