BUG: Restore stored label when selecting editor effect different from erase

Selecting the "EraseLabel" effect was storing the current label into
the 'storedLabel' parameter node attribute but this one was never restored
when selecting an other effect.

This commit fix the issue by ensuring the stored label is restored and also
introduce two new functions: 'backupLabel' and 'restoreLabel'

Fixes #3576

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22862 3bd1e089-480b-0410-8dfb-8563597acbee