Add case check on case-insensitive systems in voicebank check

[sdercolin]

Implement VB checking of duplicates like a.wav and A.wav which would not work on case-insensitive OS.

Also removes some conditions on the current OS, so we apply all checks on all platforms, without knowing whether the current OS is case-sensitive or not.

[stakira]

Could you please unify the check despite OS to ensure:

1. Case matches.
2. No files that only differ in case. We can do so for all files, not just wavs.

The check doesn't block normal uses. So let's simply be strict, so that checked VB works everywhere.

[sdercolin]

@stakira Please check again!
cc @oxygen-dioxide