FindIce: New module to find ZeroC Ice

- autodetects Ice on all major platforms
- allows building with all supported Visual Studio versions on Windows
- autodetects the slice path on most platforms
- separately detects the Ice programs, headers, slice files and
  libraries so that any Ice configuration or installation errors can
  be accurately reported, making diagnosis of Ice problems simpler
Merge topic 'FindIce' into next

050caef3 FindIce: New module to find ZeroC Ice