ENH: for issue #3002 fix eyedropper

Make sure all label effects have an instance of their logic class
and that the allow the superclass to optionally process keystrokes
and other events (this allows the eyedropper operation to work).

See keystroke list here:
http://wiki.slicer.org/slicerWiki/index.php/Documentation/Nightly/Modules/Editor#Keystrokes

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21875 3bd1e089-480b-0410-8dfb-8563597acbee