Checking date format is missing when inserting record in attribute table manager

[annakrat]

Attribute manager allows to save anything (for example integer) in the date field but reloading the values fails with error 'Unable to scan date'. This applies to sqlite but I expect to be similar with other backends. So some checking should be there before it is actually stored but I don't know what kind of format is acceptable, it depends on the backend probably.

[annakrat]

Still valid.