Refactor how the notebook changes are picked up, events bound and unbound

[krassowski]

Previously the binding of events was only performed upon the user entering a code cell. Moreover, updates to the virtual document were only triggered on code edition, and not on cell edition (rearrangement, cell pasting etc). This was kind-of ok for the initial demo, but a number of scenarios require a better management of the events binding [E] and document updates [D]:

• code cell being converted to markdown or raw cell:
  • [E] we do not want to trigger LSP there
  • [D] we want to refresh virtual document to exclude the converted fragment
• code cells being pasted or converted from markdown/raw cells:
  • [E] user should be able to get context menu on those without the need to enter them first
  • [D] the diagnostics should be shown in these just after paste
• cells being moved around:
  • [D] document should get updated to reflect the changes (and query diagnostics) associated with the change

Overall this small refactoring has a hug potential to squash numerous bugs and stability issues that arouse only after a longer work with a notebook (i.e. involving the use of feature such as moving cells, pasting, conversion of types) and were difficult to pinpoint so far.

References

Fixes #393.

Code changes

• Added editorAdded and editorRemoved events to WidgetAdapter.
• Added callback for removal of events from editors and made use of it.
• Started tracking Notebook.model.cells and trigger document updates, or editor events from there, as needed.

User-facing changes

before	after

Plus pasting of cells works better with LSP, but a third set of GIFs would be redundant.

Backwards-incompatible changes

None

Chores

• linted
• tested
• documented
• changelog entry