#484 Undo/redo retains observations across dataset loads

[David Benn]

Rob Jenkins reported this bug in which ndo/redo retains observations across dataset loads. See http://www.aavso.org/8-bug-reports-and-feature-suggestions#comment-14730

On a call, Rob described a simple scenario that reproduces the bug:

1. Load alf Ori (could be anything) with default JD range, not additive.
2. Select all obs in obs list. Exclude.
3. Load bet Lyr (could be anything) with default JD range, not additive.
4. Select all obs in obs list. Exclude.
5. Undo, Redo, Undo, Undo => alf Ori and bet Lyr obs show up in the plot but not the obs list!

I suspect that UndoManager data structures are not all being cleared on new dataset load. Also check that Edit menu items are set to default also.

[David Benn]

In 2.16.8