v1.3.0: native voices per language

Added

• Several voices per explainer: motion voice <scene> a,b,c renders each, and the player shows a voice picker.
• Native voices per language: with no voice given, Spanish narration gets ef_dora, em_alex and em_santa;
  English gets af_heart. Language comes from the scene's lang or is detected from the narration.
• A short pause between sentences in narration.

Changed

• Spanish narration uses Latin American pronunciation (es-419) by default; lang: 'es-es' for Castilian.
• motion voice refuses a voice from another language (it would read with a foreign accent) and names the native ones.
• Narrated steps move on when the voice ends, without an extra silent reading pause.