Simple multipage support for onion skin animations

A simple to develop and easy to use feature to allow svg keyframe animations. Inspired by the Snap.svg library, capable of svg elements interpolation for automatic generation of in-betweens. Details of the proposal here: https://bugs.launchpad.net/inkscape/+bug/1343411 . This would allow to edit frames and save those as separates svg files (and when svg 2.0 will be out, as a multipage svg), with a numbering appended to the file name corresponding to the frame. Both 30 fps and 60 fps speed will be supported. From the keyframe files Snap.svg will be able to generate all the in-between frames and apply easing function like Linear, EasingInOut, Elastic, Bounce, PingPong, etc.