Enable color gradients on flows

[dsb3]

I saw comments in the issues that other folks would like colour gradients available, and so would I, so I wrote it.

It's fairly complete, but does have a couple of small notes needed.

Missing is the ability to customize the colour gradient in the flow definition panel - the only way to get them is through the "inherit from" setting "source-target". I have a regex that could be used to detect the new format but no code to implement it.

When drawing very curved flows, the original rendering method (bezier curve with fixed width) looks pleasing, but the new code (filled shape with the same bezier on the top and bottom) does not. Without redrawing with a much more complex algorithm it's just going to not look as good when the curviness is close to maximum.

I've gone through enough testing to ensure it's satisfactory for my needs, but happy for feedback, comments or others testing results.

[dsb3]

Above is what I mean by rendering when "curviness" gets too high. On the left is the approximation that is a solid shape that can take a gradient fill; wheras on the right is a bezier curve of constant width.