Added a new test for SplinePlug.

I wrote this while hunting down a problem that turned out to be unrelated, but you can never have too many tests...
Added spline parameter support to RenderManShader::loadShader().

Splines are represented as a pair of shader parameters called <name>Positions and <name>Values, and which are mapped to a single SplinePlug called <name>. UI to follow.
Reintroduced SplinePlugValueWidget and SplineWidget.

SplineWidget now uses the appropriate colour transform for the display, and gets the curve the right way up (Qt origin is top left). SplinePlugValueWidget uses ramp drawing mode for colour splines.
Fixed RenderManShaderUI to take into account spline parameter name munging.