Allow color to be explicitly compared to a string

Allow color to be explicitly compared to a string

Add the Qt.colorEqual() function which compares any combination of
two supplied color and string arguments, by converting the string
arguments to colors as necessary.

Task-number: QTBUG-18754
Change-Id: I75baef9a2edd30a5f8b9cb5e151e4adba6f6a371
Reviewed-by: Michael Brasser <michael.brasser@nokia.com>