`mbtn_left` and `wheel` events are now only fired on a `cursor.on_{event}` object, which resets at the beginning of each frame, so elements need to bind these listeners in their render functions. These properties are overwritable which allows elements to take over cursor event handling from their parents if necessary.
I don't like this solution, but anything cleaner would probably require converting chapter indicators into individual elements and than managing/updating their positions and sizes in relation to growing timeline, which seems like a lot more work and code, so this'll have to suffice.
closes#370
