If you have some small windows, it's very obvious that when you
go from 1 workspace to 2 workspaces:

 - The windows are left too small

And when you go from 2 workspaces to 1 workspace:

 - The windows are left too big and will be scaled up beyond
   their original size

This patch fixes it by exporting positionWindows from Workspace
and calling it at the appropriate time. Alternative would be
to have workspace.setScale() rather than have have Workspaces
set workspace.scale directly - then the Workspace object could
call _positionWindows as appropriate.

That's maybe a little cleaner but some care would have to be
taken to not have the code start multiple animations.