Cell "executing" state

[davidbrochart]

Currently, the execution_count field of a code cell can be None or an integer greater than 0, the former meaning that the cell was not executed, and the latter meaning that the cell was executed. But there is no way to denote that the cell is being executed, although the notebook could be saved while in that state (for instance when a cell takes time to execute). Actually, since outputs can be generated while the cell is executing, if the notebook is saved during that time, the cell will have outputs but it will appear as "not executed", which seems contradictory. At the same time, frontends such as JupyterLab show this information with a * in the execution_count. I think it would make sense to allow this value to be stored in the execution_count field of a code cell. See https://github.com/jupyter-server/jupyter_ydoc/issues/169 for a motivation about using this in the context of RTC.