Clean up cell menu

[doolittle]

We don't need all of the cell menu actions and could possibly do away with it.

Here's the current list:

• Moving up/down could be removed by making the cells draggable.
• Switching modes could be removed (not sure anyone would do it)
• Deleting a cell would be accomplished by pushing backspace after you've already cleared the contents of a cell
• New cell (and insert API) is handled with the "insert cell" dot
• Execute could be handled by using the menu space as a "play" button that would execute the cell.