[FEAT](Extension Scenes Legend Item) New kind of extension points: Scene Variables extension points.
Allow overriding the calculation of CCC scene variables. This is useful for things like changing the text of discrete ticks or legend items in a way that layout takes it into account. Normal "axisLabel_text" and "legendLabel_text" extension points are not taken into account by the automatic layout algorithms because they cannot be safely used in a context were protovis marks do not yet exist (as is the case). These are not suitable to be used when the size of panels is determined automatically.
In practice, for now, there's a single new extension point: "legendItemScene_value" (or "legend2ItemScene_value", ..., and "legend$ItemScene_value", for any legend group) that allows changing the label of the legend item's scene variable 'value'.