Win32 MIDI driver installer improvements:
- Bugfix: added missing deletion of the newly created temp file
- Error handling changes:
  - installation path length is now checked and if it is too long, the installation is aborted
  - if the driver cannot be registered, the driver file won't be installed in the system
  - the driver can now use MIDI driver registry entries filled with empty strings
  - if there are multiple wdmaud.drv entries found in the registry, the driver can now use the second found entry
  - error message is now displayed in case of file copying failure
- Other minor cleanup