ENH: Added icon loading for scripted module base class

Python scripted modules that use the ScriptedLoadableModule base class
now load their icon from Resources/Icons/<ModuleName>.png.
If the icon file does not exist then the default icon is kept.
Modules can override the icon setting in the module __init__ function to
use any other custom icon.

Updated extension wizard template and extension test.

Updated one more module (VectorToScalarVolume) to use
ScriptedLoadableModule base class.

From: Andras Lasso <lasso@queensu.ca>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23676 3bd1e089-480b-0410-8dfb-8563597acbee