Plank bounce animation should not include the icon highlight

## Description

The current bounce animation when opening new windows on an already open application will also make the icon highlight jump. I think this looks unnatural and awkward. Please watch the attached screencast for an example of this.

## Proposal

Change the animation so that the highlight "stays on the ground" and only the icon itself jumps.

Thanks for reading this

## Additional information

$ policy plank
plank:
  Installed: 0.3.0+bzr872-0ubuntu1~12.04~ricotz1
  Candidate: 0.3.0+bzr872-0ubuntu1~12.04~ricotz1