Use only one effects attribute instead of effectIn and effectOut

Instead of using an "effectIn" and an "effectOut" attribute, it would be more flexible to just use one "effect" attribute. Information about the different effects could be stored using a JSON-like syntax. This would for example allow to assign an arbitrary number of effects to each object. It might be good to save some new information (e.g. whether the object is visible at the beginning and end of the effect) with the effects, so that the Inkscape extensions can perform basic checks, whether the effect sequence is consistent.