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