Update cell and breakpoints on content changed

[jtpio]

For the debugger to work, we need keep the content of the cell in sync with the kernel. This is because of the hashing mechanism detailed in #49 and #52.

That means that every time a user modifies the content of the cell, we need to send the dumpCell and setBreakpoints requests, for the debugger to be able to break at the next execute_request.

The flow would be something similar to the following:

• Add a signal connection to activeCell.model.contentChanged.
• When a user changes the content of the cell, the signal is emitted.
• Send dumpCell and setBreakpoints via the DebugService, probably behind an ActivityMonitor or Debouncer to avoid firing to many requests.
• Since the service is responsible for updating the breakpoints, the breakpoints view will refresh itself when a response is received from the kernel and the model changed.

This was also raised in #160. Before the breakpoints model would be directly modified when the number of lines of the cell changes (and the service bypassed).