Gradient Builder fluent API

[mgierlasinski]

Provide fluent API:

var gradients = new GradientBuilder()
    .AddLinearGradient(g => g
        .Rotate(20)
        .Repeat()
        .AddStop(Color.Red, Offset.Prop(0.2))
        .AddStop(Color.Blue, Offset.Prop(0.4)))
    .AddRadialGradient(g => g
        .Circle().At(30, 30)
        .OfSize(200, 200)
        .StretchTo(RadialGradientSize.FarthestSide)
        .Repeat()
        .AddStops(Color.Red, Color.Green, Color.Blue))
    .Build();

Extensions for GradientBuilder:

gradientView.GradientSource = builder.ToSource();