Windows: Add a context menu for the AFS icon

Windows: Add a context menu for the AFS icon

The OpenAFS plug-in for Network Identity Manager displays an icon
in the notification area that displays status information about
the AFS service and tokens.  Add a context menu to the icon so
that users can open the NIM application and get help for the
plug-in from the notification icon.

Change-Id: I704934b4f4eab72a18c54ef2a7a7dc3ca248ba78
Reviewed-on: http://gerrit.openafs.org/1121
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>