bugfix: allow both SSSML <audio> and soundicon punctuation
All sound file information goes to soundicon_tab. The contents of the files are not
loaded to memory before they are needed for synthesis.
This removes N_SOUNDICON_SLOTS that divided soundicon_tab into two
parts for unknown reasons.
TODO: LoadConfig() is currently undocumented. The logic should be
moved to LoadVoice() and documented.
Soundicons are used for external audio with SSML <audio> tag and for
replacing punctuation names with sound files in LoadConfig().
Currently there's a bug wih soundicon slots: if both LoadConfig and
<audio> are used, the punctuation reserves all slots and no sound from
<audio> is played.