Implement new Windows installer based on WiX toolset.

This implements a build system-compatible approach to constructing Windows
MSI installers, based on the WiX toolset.

* Uses the WiX minimal wizard dialog set; this is serviceable although not
  particularly pretty.
* Provides the GNU GPL license for the dialog set in license.rtf
* Adds static compilation options for Windows build to remove dependency
  on mingw DLLs.

Further cosmetic refinements are likely indicated, but this should establish
the basic framework required.