Set our explicit AppUserModelID on our Start Menu shortcut.

This is apparently the other half of what we should have done when we
called SetCurrentProcessExplicitAppUserModelID at run time: it
associates to PuTTY's Start Menu shortcut the same identifier that we
give to the running PuTTY process, so that jump lists saved under the
latter will be visible to users mousing over the former.

I've also done the same thing to the desktop shortcut, just in case
that does anything useful.