STYLE: Rename Slicer(Wizard.py) -> Extension

Given its current functionality, SlicerWizard.py is more appropriately
the Extension Wizard; rename it accordingly. This also allows us to
shorten the actions (by dropping 'extension' from the end).

Also, the 'submit' action has been renamed to 'contribute'.

Note that the module code has NOT been moved, as it conceivably could be
used by some other script. (Although the SlicerWizard class is renamed.)
Thus, the driver script now says 'from SlicerWizard import
ExtensionWizard'.