mixer: move members to initializer part of constructor
mixer: rename NumFxChannels to NUM_FX_CHANNELS to hint global variable.
mixer: add stubs of MixerBuilder
mixer: move fabrication logic to MixerBuilder

This fills in the build, buildFrames, buildFxChannels, buildFifo stubs in
the MixerBuilder, with the previous statements in the Mixer constructor.
mixer: move aligned/mal/free stuff to MemoryHelper
memoryhelper: missing ;
memoryhelper: add stdlib.h
mixer: move rest of fabrication logic to builder
mixer: make engine use the builder now