Refactoring of the ALSA part

src/audio/alsalayer.cpp

src/audio/alsalayer.h

src/audio/audiolayer.cpp

src/audio/audiolayer.h

src/audio/audiortp.cpp

src/audio/pulselayer.cpp

src/audio/pulselayer.h

src/eventthread.cpp

src/iaxvoiplink.cpp

src/managerimpl.cpp

src/samplerateconverter.cpp