Implement Notebook Cell Semantics

[blink1073]

Notes:

• New cells are always code cells and are started in edit mode
• Executing a cell selects the next cell but does not focus it
• If the cell is not focused, we are in command mode
• Cells can be executed in command mode
• A cell can be: selected, focused, and marked, each of which has a different semantic meaning
• Rendering an empty markdown cell should display a message:
  Type Markdown and LaTeX: α^2

[blink1073]

cc @jasongrout @Carreau @ellisonbg

[blink1073]

cc @minrk

[Carreau]

Executing a cell selects the next cell but does not focus it

that's only with the keyboard. Ctrl-Enter execute and select same cell.

The text in empty markdown cell is not that strict as long as it guide users IMHO.

[minrk]

Yup, I think that's the gist. As @Carreau mentioned, the exact placeholder text isn't important. It's mainly there to keep a rendered, empty markdown cell from being invisible.