preserving conditional compilation

[atsushieno]

Conditional compilation is useful when we want to start from anywhere in the song, but it brings in "bugs" in MML in that such conditionally compiled tracks especially with #conditional block results in unexpected velocity, octave, key shifts etc. No one wants inconsistent song outputs depending on whether conditional blocks are used or not.

A solution is to filter out only MIDI note on messages.

[atsushieno]

We cannot simply treat existing conditional blocks as in the model above because existing blocks may be more like "pattern candidates the composer can choose i.e. pick one of them". For such blocks, the resulting song file should not even contain non-note messages.

Therefore, there should be another kind of #conditional indicators.