Convince SWIG that Color.r (g, b, a) should be a python property

While %attribute is not mentioned in SWIG documentation, there is
https://github.com/swig/swig/blob/master/Lib/typemaps/attribute.swg

Instead of using the old getR(), getG() getters and setB(128), you now
can do:

    >>> c = fife.Color(80, 0, 255, 128)
    >>> print c.r  # This replaces getR()
    80
    >>> c.g = 64   # This replaces setG(64)
    >>> print c.g  # This replaces getG()
    64

and so on. Note that the `Alpha` setters are rather likely to become
deprecated soon in favor of `getA()` and `setA()`, so the name of the
associated python property also is `a` over `alpha`:

    >>> print c.a  # This replaces getAlpha()
    128
    >>> c.a = 255  # This replaces setAlpha(255)