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.