Add synchronization points between tracks

[krzema12]

Using the DSL, it's a common problem to adjust the number of repetitions of a certain piece to match e. g. the main melody. It's currently hard-coded and it's hard to understand what this number means:

https://github.com/krzema12/fsynth/blob/06cde24e9e898b4d94a9e6c5b3b65854873da4f0/core/src/commonMain/kotlin/it/krzeminski/fsynth/songs/PinkPantherThemeIntro.kt#L156

There could be named synchronization points, set by one track and used by the other, to determine the number of repetitions.