track: Avoid various sorting operations before timeline commit

These are showing up in performance profile of 1000+ clips looped addition. All this is done at commit time as well, so let that do only one update and sorting.

This was part of !62 but the other changes cause various integration tests fallout, while this bit is what gains the most performance for batched clip addition and looks like it doesn't even need any test adjustments, thus may be safer for 1.16.x as well.