Separate (background) music & sound effect

[nighca]

In SPX and Scratch, "sound" usually refers to two scenarios:

• Sound effects
• Background music

Separating music from sound effects offers several advantages:

• Music is typically played in the background for extended periods, often using loop, pause, and resume features.
• Sound effects are usually triggered briefly to signify game events, without looping or pausing.

By distinguishing these two sound categories, we can:

• Enable background music configuration for each scene.
• Simplify sound control APIs, such as:
  • Non-blocking API only for music
  • Blocking API only for sound effects
  • Removing loop, pause, and resume options for sound effects
• Allow separate volume control for music and sound effects.