It doesn’t look like I’ve done much, but the whole layering architecture was changed

Now the layers support vertical scrolling

The new architecture

1. I place the playback controls at the top, then I split the remaining part horizontally.
2. A vertical scroller contains the hierarchy of the layers in the left part. The right part contains a horizontal scroller that contains the tick bar
3. The horizontal scroller contains another vertical scroller linked to the one on the left

The aims were to allow vertical and horizontal scrolling on the right, but keeping the left panel and the tick bar visible. Now I have the layers_ vector, the tick bar, and the time cursor all inside the Timeline class, instead of the odd spaghetti way I did it before.