Keyboard commands don't check for editability before performing editing commands.

[Connor Carney]

We need to run all editing commands (enter in headings, indenting lists, navigating tables, etc.) through a single dispatcher that handles *all* of the keyboard shortcuts, that way we can check for editability just once before editing.

[Connor Carney (cscarney)]

The issue with the paragraphs is fixed, but the larger issue isn't fixed -- there is exactly the same problems with lists, and tables, and pretty much every editing feature we add in the future.

Right now, each editing feature registers its own keypress handler, then checks the context and performs the appropriate modifications. We need an infrastructure for scripts to register a combination of key, context, action, then have a single keypress handler where we can check the context *once* and dispatch it to the appropriate handler.