Refactor - Improve the meaning of org-get-entry in the context of
opposing card and checkbox
  - [X] 'Sync card to trello' syncs card only if local changes
  - [X] 'Sync card from trello' updates the card from trello and the local checksum signature
  - [X] 'Sync buffer from trello' updates the local card checksum
  - [X] 'Sync buffer to trello' syncs only card with local changes
Refactor - Remove dead code
Refactor - Rename checksum variable
  - [X] 'Delete entity' updates the checksum
  - [X] Extend the checksum system to the checkbox entities (checklist/item) - sync to trello