ENH: Tweak Multi-sampling option

This commits improve the user experience related to
new multi sample option introduced in r23305. It now
allows to specify the number of samples.

Finally the code has been refactored to improve
the flow of changes and to use a cleaner signal/slot
interface.

From: Eric Larson <larson.eric.d@gmail.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23333 3bd1e089-480b-0410-8dfb-8563597acbee