Add path introduction

[jakubg1]

Path introduction is a trail of particles going through the entire path before spheres roll in. Example config in gameplay.json:

"pathIntroduction": {
    "particle": "path_introduction.par",
    "speed": 1500,
    "density": 24,
    "separatePaths": false
}

• particle - The particle that is going to be spawned.
• speed - How fast the particle trail should move along.
• density - The distance between each particle.
• separatePaths - If set, each path will be introduced immediately after another one finishes. Else, all paths will be introduced simultaneously.

To disable path introduction, just omit this section. This also means that this will be a non-breaking change.

[jakubg1]

Has been implemented some time ago: https://github.com/jakubg1/OpenSMCE/commit/04d73353ad5bae3ef2db8c8d4a983394dbfad6be