Mass labels in Motion views can get misaligned

[jessegreenberg]

Found while testing for #226, the mass labels on PhET girl and man in the motion screens can get misaligned. Not caused by the changes in #226, this can be reproduced in the deployed version, from which this screenshot was taken:

Steps to reproduce:

• Place PhET girl on item stack
  • Apply force on item stack to the right so items face right
  • Stack more items in the stack until PhET girl is moved back into the toolbox

Something about the images changing orientation from right to left is causing this.

[jessegreenberg]

Fixed in the above commit, closing.