PERF: Improve model update performance

Model module's widget computed model surface, volume, etc. every time the model changed, even when the module info section was not visible.
This cause a significant overhead when replaying changing models using Sequences module.
Applied a similar technique as for transforms info: only compute model properties when the window content is actually visible.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25386 3bd1e089-480b-0410-8dfb-8563597acbee