send targets are now by ID and Song has "Score" part, which is the notes for it. also, moved the model part separate of the actual gioui dependend stuff.
sorry to my future self about the code bomb; ended up too far and did not find an easy way to rewrite the history to make the steps smaller, so in the end, just squashed everything.
assume songs code it as 1 always; implementations are free to change this during compilation, but this should be a compile time flag / optimization; not a concern of song.
Play depends on bridge and compile on compiler package. Before, the compiler depended on bridge, but we could not use the compiler to build the library, as the bridge depends on the library. Also, play can now start having slightly more options e.g. wav out etc.
