Added fading gradients through Squirrel

[Semphriss]

Added four functions through Squirrel to fade colors over time :

// In addition to set_colo1 and set_color2, set_colors allows you to set both colors in one function
set_colors(float red, float green, float blue, float time);

fade_color1(float red, float green, float blue, float time);
fade_color2(float red, float green, float blue, float time);
fade_colors(float red1, float green1, float blue1, float red2, float green2, float blue2, float time);

[Semphriss]

Note : I'd like someone to review my coding style, if possible; in gradient.hpp I declared object properties as private and starting with "m_"; is that the correct/recommended way of proceeding? How is it best to do?

[tobbi]

I thought we already had that? I'm a bit confused.

[Semphriss]

I thought we already had that? I'm a bit confused.
@tobbi

There was an option to change the color instantly (set_color), but not to fade progressively over time. This PR adds this small missing feature :)