Add buttons for x1.5 and /1.5

[Jirinrin]

I've noticed that the automatic BPM estimation algorithm often gets it wrong by a factor of 1.5 (so it basically counts the "thirds" in the song). For instance, for a set of house songs, about 30% of them came out around 80~90 BPM, and multiplying them by 1.5 made it match the song perfectly.

By the way, I don't have the optimism that anyone is actually going to implement this, but I'll leave this issue anyway as a bookmark for myself if I ever want to clone this project and to try implementing the feature myself :)