Style improvements

Use PWSTR instead of LPWSTR
Use .def file instead of linker #pragma comments
Fix memory leak
Significant impl changes, no user impact

* Don't bother checking for an explicit process AppUserModelID
* If a shortcut exists, get its [probably autogenerated] AppUserModelID
and use that
* Otherwise, create a shortcut with an explicitly set one
* Also add a method to get the current ID--useful for debugging