Multichannel .wav should be saved as WAVEX

[Robert Tari]

By default, multichannel and high-resolution wave files are currently being saved with the simple WAVEFORMATEX header, which is no longer sufficient.

For files like these, the WAVEFORMATEXTENSIBLE header should be used, i.e. the application should default to the "WAVEX" option in the file save dialog when it is auto-detecting from extension.

A valid channel mask should also be set (3f 00 00 00 for 6-channel audio, 37 00 00 00 for 5 channels, etc.)

Tested on mhWaveEdit 1.4.23