* all: support of animated bitmaps in skins: there are new attributes

* all: support of animated bitmaps in skins: there are new attributes
  "nbFrames" and "fps" in the (Sub)Bitmap elements to set the number
  of frames and the frame rate in a bitmap. As in the radialslider,
  the different frames are just laid vertically in the bitamp (and
  all the frames must have the same size)
  At the moment animated bitmaps are only supported in Button controls.