Fix DataTable crash when removing and updating cell at the same time

[darrenburns]

As per title. There was a crash when you tried to remove a row/column and also update a cell within a short period.

• The logic for updating dimensions after removing a row runs in on_idle.
• When you tried to update a cell that was in this now removed row on_idle wasn't informed that the cell was removed in the time since check_idle was called.
• Now we update the data structure that on_idle uses to know what cell dimensions to recompute before it runs.

Please review the following checklist.

• Docstrings on all new or modified functions / classes
• Updated documentation
• Updated CHANGELOG.md (where appropriate)