Keep all the breakpoints of a notebook in the model

[JohanMabille]

The breakpoints model should keep the list of all the breakpoints of a notebook instead of those of the active cell only. Internally, whis can be achieved by storing a map of list, the keys of the map being the hash of a cell computed with the same method as the one for duming cell in the backend.

This has many advantages:

• no need to resend the breakpoints to the backend when we switch from one cell to another
• no need to reset the breakpoints to an empty list and then retrieve the breakpoints of the new active cell from the editor
• breakpoints are restored thanks to the restore service when the user switches to a different notebook / console.

Besides limiting the number of messages that are sent (which is not a prformance issue but rather a debugability issue), it would simplify the internal logic.