BUG: Fixed editor behaviour in restoring correct label.

To reproduce the issue:

• Load a volume, go to editor and create a merge labelmap.
• With Paint effect paint something with label 1.
• While on Label 1 select eraser, erase something and go back to painteffect.
• Select another label e.g 2.
• Paint something with painteffect. It works fine (paint with label 2).
• Choose another effect (e.g draw or want effect) and it would go back to the label 1 as it only stores the backup label when eraser is selected.

Fix: Backup label should also be set when a new color (not zero) is chosen not only after switching to eraser.

From: Alireza Mehrtash <mehrtash@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24108 3bd1e089-480b-0410-8dfb-8563597acbee