Create sound infrastructure in the emulator

[lgblgblgb]

Create the sound infrastructure, keeping in sync, etc, as a test, implement D/A mode only, later Dave audio generation as well (this one is probably another issue ticket).

[lgblgblgb]

Note: with addition of support for all Dave interrupts, TG channels are emulated now but without any "effect" (modulation, distortion etc). It can be used later to start to develop the audio generating as well other than the D/A for sure.

[lgblgblgb]

Audio init, stop/start code fragment is implemented but not used for playback yet. Commit 1c5a581

[lgblgblgb]

First try with real audio. Very dirty, ugly hack, without buffer over/under-run protection, inaccurate, etc, but it does something at least :-/ Note, audio_init(0) in main.c disables it, you should use audio_init(1) to try.
Commit f942616

[lgblgblgb]

Audio can be tested in released win32 build too, with :XEP AUDIO command.
Commit 7ad0514

Still, the infrastructure is pre-alpha quality no sync, etc, anything :(

[lgblgblgb]

Minimal SDL version is 2.0.4 now. There are new possibilities with this SDL version, ie audio queuing. Should be used ... With maintain sync in timing code as well.

[lgblgblgb]

Though it's a Xep128 transferred issue, can be meant to the Xemu project as well, still. However there is already an issue for similar problems in Xemu: #90

[lgblgblgb]

So closing this one for now ...