Minimum audible loop size was too aggressive, make it longer.

Tested with extremely short buzzy loops.
Avoid heap allocation of SoundSourceProxy and fix memory leak
Replace plain pointer with QScopedPointer
New function SampleUtil::inPlaceMonoToStereo
New function SampleUtil::convertS16ToFloat32
Avoid heap allocation of struct member
Fix undefined references errors temporarily
Cleanup members and constructors of various SoundSources
SoundSourceProxy is-not-a SoundSource
Remove TagLib dependencies from SoundSource base class
New API and base class AudioSource
Preliminary implementation of the new AudioSource API
Partial migration to new AudioSource API
Remove length() and seek() from SoundSource API