ENH: Create new slicer "wizard" tool

Create new SlicerWizard.py tool, more or less based on the existing
ModuleWizard.py, with improvements. This one separates the concepts of
extensions and modules, and allows creating an extension containing
several modules, as well as adding modules to an existing extension.

The mechanism for specifying templates is also changed; we now take
named templates which are searched for in a well-known location relative
to the script. (In the future it is expected that users will be able to
add additional template directories, however this has not been
implemented yet.)

Eventually it is hoped that this tool will add other actions, e.g.
publishing extensions.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22946 3bd1e089-480b-0410-8dfb-8563597acbee