Add "Animation" tab (instead of group) for joints

This makes it consistent with ''Modelica.Mechanics.MultiBody''
Merge pull request #52 from tbeu/add-animation-tab

Add "Animation" tab (instead of group) for joints
Merge branch 'master' of https://github.com/fvanderlinden/PlanarMechanics

Conflicts:
	PlanarMechanics/Joints/Revolute.mo