Feature: Arranger - Ability to alt-click split arranger sections

[Josh Wolfer]

Exactly as the title states. Probably a slightly non-standard use for the arranger track, but I'm doing film sound editing and I like to use the Arranger track to mark big scene changes. It would be awesome if I could split an arrangement section into 2, by alt clicking, just like you do with clips in track view.

You can already right click -> Split at now time, so this is just making a faster way to do that.