[Inkscape-devel] SPStyle now uses SPFilterReference

I changed SPStyle to use SPFilterReference for linking up filters, the same as filters use to link up to each other. This simplified SPStyle code (got rid of ad-hoc hreffing, etc) and fixed at least one bug: now filters are no longer lost when you undo an effect from Effects menu. Of course there may be gotchas, so please report to me any crashes and misbehaviors in filters linking (but not rendering - that is Niko's domain now, my changes should not affect that). If all goes smoothly I will similarly switch SPStyle's paintservers (gradients and patterns) which will fix a bunch of other long-standing bugs.