Add incoming notes without shifting scroll position.

[jb55]

Now that we have a bit more control over scrolling, we have an opportunity to do something really cool.

Instead of queueing notes to be inserted, we can simply render all of the new notes, calculate their sizes, put them in the timeline, and then adjust the scroll position accordingly. This will allow us to load content in realtime without moving our scroll position.

[alltheseas]

@robagreda how might the design look like for this? For instance, say that there are newer notes loaded - what sort of UI affordance could communicate there are new notes?