Prosemirror cell switching

[stefnotch]

The cell switching currently doesn't support prosemirror #29

Prosemirror itself doesn't have a concept of line numbers, instead it's tree shaped (like the browser dom) and leaves most arrow key navigation things to the browser. Prosemirror uses contenteditable under the hood.

This makes figuring out if the user is at the top of the document rather tricky. For example, imagine the cursor being placed after the <br>a. Then, while the cursor is in the topmost element, it's not at the top of the document. There is still a <br> before it.

With some work, we might be able to cover all possible cases and implement arrow key navigation. One other approach would be to add a zero-height div at the top of the document. When it gets selected, the cell switching gets invoked.

One alternative approach would be to, when the user presses the up arrow key

1. before anything happens, save the cursor position
2. after the event is over, get the new cursor position and compare it to the old one