Allow Converting of Non-Single Channel Samples .wav to Single Channel

[RobDavenport]

Currently, importing samples for the sampler instrument just takes everything and treats it as a single channel. That means for dual channel samples, they end up playing back at half-speed.

This forces users to manualy convert their samples to mono, which may require special tools or software.

Since we have all the data, we should have a way to do this, either by using a single channel (left or right), or maybe combining the two channels into one.