Add ImageJService to PTService subclasses

All PTService subclasses were erroneously SciJavaServices as well.

SciJavaService has been removed as a superclass of PTService, so all
PTService subclasses must now align themselves with a different
"identifying" service.

Thus, all PTServices should now be marked as ImageJServices instead, to
ensure proper discovery scope.
WidgetModel: tweak comment
Merge branch 'mark-imagejservices'