SampleUtil Enhancements

Enhancements and extensions of SampleUtil. Some of the new functions are currently not used, but will be when the new SoundSource API follows. I adopted the naming "widen"/"narrow" for the conversion of signals between a different number of channels: mono --"widen"--> stereo <--"narrow"-- multi-channel

Some minor issues are fixed in separate commits.

hm, no that one has been around :)

Ugh, not sure why mixxx.org is down. Looking into it. Meanwhile, Google cache has it:
http://webcache.googleusercontent.com/search?q=cache%3Amixxx.org%2Fwiki%2Fdoku.php%2Fcoding_guidelines&oq=cache%3Amixxx.org%2Fwiki%2Fdoku.php%2Fcoding_guidlines&aqs=chrome..69i57j69i58.6758j0j7&sourceid=chrome&es_sm=91&ie=UTF-8