ENH: slicer.util: Add NodeModify context manager

This commit adds a convenience context manager allowing to easily
disable modified event associated with a node, and automatically re-enable
them and invoking them if it applies.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23634 3bd1e089-480b-0410-8dfb-8563597acbee
ENH: EditBox: Simplify observation introducing the use of VTKObservationMixin

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23635 3bd1e089-480b-0410-8dfb-8563597acbee
ENH: Editor: Effects are checkable and updated based on the parameter node

This commit ensures the button effect are checked or unchecked based
on the 'effect' property stored in the parameter node.

It also extends the EditUtil API adding function allowing to more easily
set and get the 'EraseLabel' state.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23636 3bd1e089-480b-0410-8dfb-8563597acbee