BUG: Enable numeric sorting in LabelStatistics table

Data is added as float in a QtVariant with Qt::DisplayRole, in stead of text. The
enable the columns to be numerically sorted.

From: Bradley Lowekamp <blowekamp@mail.nih.gov>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23003 3bd1e089-480b-0410-8dfb-8563597acbee