Add Dlg_*_Init callback to dialogues. To be used e.g. for starting timers of NPC activities.

Also fix scenario saving of attention state in dialogue object.
Merge all of the audio provider switches into one

There's no point in splitting the audio library selection into multiple
CPP macros, since there can always only be one anyway. Merge all of them
into a single macro AUDIO_TK (for "toolkit") and have CMake select one
for the user, instead of making him choose (and potentially failing).