Fix icon loading on Windows, closes #734399

Create an index.theme file to make sure Gtk.Theme.LoadIcon() finds the file/we get icons on Windows
Change Setup project to
- build with a standard WiX install (use the right MSBuild path, the WiX defined MSBuild variables in references)
- include the index.theme in the MSI
- remove empty/useless gtk icon theme folders

Signed-off-by: Jared L Jennings <jared@jaredjennings.org>