Node for constructing a Gradient

[0HyperCube]

The fill node accepts the Gradient type. This contains the start position, the end position, the linear/radial, and the stops.
The «gradient value» node creates the GradientStops type which only contains the colours and the fractions.
This makes it impossible to actually use the fill node from the graph, preventing the easy sharing of gradients between layers.

There should be a new node to construct a Gradient using the stops and the other data.

[Keavon]

Work on this should be towards the goal of what's described in #2875.

[adamgerhant]

The fill_properties, FillChoice, Fill, Gradient, GradientStops, and ColorInput widget are all a mess, and its not simple to add the widget for the Gradient tagged value

[Keavon]

Yes, it'll need rather an overhaul, one I've been planning for a month or so. I can go into more detail about the plan if/when necessary.