Orignally requested by @SockHungryClutz

BPM change keyframes should be added to the level editor. As previously discussed in the contributor chat, this should come alongside a UI update to move the keyframes out of the way of notes.

A few suggestions for how to implement the UI could be to have text with the BPM value on the keyframe icon. The value could be adjusted by using the context menu with an option to change the BPM, or by double clicking the keyframe for the BPM change and having the text box be editable.