Show most recent time for cells in a condition, not the date when they w...

...ere added

Save error cell in history, to show what has occurred
Regression from a9163a5: isError could be set without errorString
Cleanup "private" add/remove for Cell Preferences

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Discussed in issue #30, partly in pull request #38
Not all changes I planned, have not seen enough error cells (except those I simulated) to activate the skipping of error cells
Also planned to switch from AraayList to HashMap and save only cell id for conditions, should maybe be done before next public release, but I want to get this for review first.