Add 'razor' tool to split clips

[mrharpo]

Create a toolbar in the timeline, and make two selectable tools:

pointer - regular cursor, for resizing edges and drag resorting.
razor - razor cursor, for splitting clips.

Clip.js

onclick: (vnode) => {
  if (vnode.style.cursor == 'razor') {
    // split clip at mouse point
  }
}