Smooth animations and no flashes for trails view

This does two big things:

- Smooths out the open/close animation (done by keeping the layer always layer-backed and animating the transform instead of the `top` property, so there's no re-layout)
- Eliminates the flash of content when the view swaps between normal and fullscreen mode (done by keeping touch scrolling always on, so it stays layer-backed, but preventing scrolling when not in fullscreen via event handlers)

Note the second commit here is a little messy; it's more a proof of concept to demonstrate a way to fix #52.