Display breaks on editor timeline & allow manually adjusting their duration

[bdach]

First step for https://github.com/ppy/osu/issues/14330

https://github.com/ppy/osu/assets/20418176/c575e202-5e08-40d7-883a-827241501b4f

This is a half-baked diff:

• No way to add breaks
• Breaks don't add automatically
• Undo doesn't work properly

but the code size has already grown by quite a bit so I wanna get this one out as a first step at least. Aforementioned points will be addressed in future pulls.