BUG: Ensure label can be changed if it was been toggled.

Following commit r22862 fixing issue #3576 to ensure label is restored
after switching effect, the label couldn't be changed after it being
toggled.

This commit now ensures that label is reset when eraseLabel has been
enabled and effect is switched.

See #3576

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22865 3bd1e089-480b-0410-8dfb-8563597acbee