Unlike wxGridCellDateRenderer, which already did it, the editor class
always got the cell value from the table as a string, even if the table
supported returning the dates directly.
Fix this by using the same code in the editor as in the renderer, which
required a further refactoring in order to make it reusable: the helper
TryParseDate() was replaced with TryGetValueAsDate() and DateParseParams
was added to allow overriding the arguments passed to it in the
overridden wxGridCellDateTimeRenderer::GetDateParseParams().
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