Move some of the audio provider machinery to libaegisub

Makefile.target

libaegisub/Makefile

libaegisub/audio/provider.cpp

libaegisub/audio/provider_convert.cpp

libaegisub/audio/provider_dummy.cpp

libaegisub/audio/provider_hd.cpp

libaegisub/audio/provider_lock.cpp

libaegisub/audio/provider_pcm.cpp

libaegisub/audio/provider_ram.cpp

libaegisub/include/libaegisub/audio/provider.h

src/Makefile

src/audio_controller.cpp

src/audio_controller.h

src/audio_display.cpp

src/audio_display.h

src/audio_karaoke.cpp

src/audio_karaoke.h

src/audio_player.cpp

src/audio_player_alsa.cpp

src/audio_player_dsound.cpp