BUG: #3012 eraser no longer cancels active editor effect

Also undo/redo.  Now you can either use the keyboard
shortcuts or the buttons to perform these actions
and the currently active tool will not be canceled.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21977 3bd1e089-480b-0410-8dfb-8563597acbee