Intuitive editor keyboard interactions

[rtfeldman]

Although the editor is semantically sectioned, navigating around it with the keyboard ought to be easy using the arrow keys (like it is in monolithic text editors). You shouldn't get "stuck" in sections, where you feel like you have to resort to scrolling to escape.

• [ ] If the cursor is at the beginning of a contentEditable, cmd+up should scroll to the top of the document. (Ditto Home and ctrl+pageUp on keyboards that have it.) Likewise, if the cursor is at the end of a contentEditable, pressing cmd+down/ctrl+pageDown/End should scroll to the end of the document. In this way, you can press Home once to get to the beginning of a section, then Home again to get to the beginning of the document.
• [ ] If the cursor is at the beginning of a contentEditable and you press Up, it should focus in the previous contentEditable. Likewise, if the cursor is at the end of a contentEditable and you press Down, it should focus in the next contentEditable. (Ideally the rule here would actually be "you're on the first line" and "you're on the last line", but the implementation difficulty might not be worth the extra intuitiveness there. It's worth trying though.)