Various API tweaks

[filiph]

Description

Sorry for the unordered bag of tweaks.

• All time-related parameters and return values are now Duration type. Before, they were double.
• Fixed velocity computation bug in example/.
• Renamed SoundEvent to SoundEventType. Quick fix available.
• SoundProps.soundEvents is now a Stream, not a StreamController
• SoundProps.soundEvents stream is now closed automatically when SoLoud.disposeSound() is called.
• SoLoud.activeSounds is now an Iterable instead of a List (therefore, it cannot be modified from outside the package).
• Renamed SoLoud.getFxParams to SoLoud.getFilterParams. Quick fix available.
• Renamed SoLoud.setFxParams to SoLoud.setFilterParams. Quick fix available.

Type of Change

• [ ] ✨ New feature (non-breaking change which adds functionality)
• [x] 🛠️ Bug fix (non-breaking change which fixes an issue)
• [x] ❌ Breaking change (fix or feature that would cause existing functionality to change)
• [x] 🧹 Code refactor
• [ ] ✅ Build configuration change
• [ ] 📝 Documentation
• [x] 🗑️ Chore