Manage read only fields in TObjectDataSet

[Inkvizi]

If I have read-only business field on form - it would be perfect to control this on object level by create property without setter. But this will generate exception in TObjectDataset. Suggest to check prop.IsWriteAble before write to prop. Becouse using fields instead property is not transparent in my opinion.

[Stefan Glienke]

set field to ReadOnly if property is not writable (fixes #276)

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