* fix tracingtool register method:   buttons should not be recreated when the tool is registered   from unregistered state as would happen when focusing   the stack from a non-stack window
* fix tracingtool register method:
  buttons should not be recreated when the tool is registered
  from unregistered state as would happen when focusing
  the stack from a non-stack window

* added 'tooltip' tags to the buttons

fzadow/CATMAID,fzadow/CATMAID,htem/CATMAID,fzadow/CATMAID,fzadow/CATMAID,htem/CATMAID,htem/CATMAID,htem/CATMAID