QDateTimeEdit accept only one format

The only date format accepted by QDateTimeEdit is the one defined in QDateTimeEdit::displayFormat. It would be more practical to be able to specify mutliple format.

For example, an user should be capable of writing 20/10/84 to have 1984 and 20/10/1784 to use the specified century.

A more general way to solve the problem is to introduce formater (like on Cocoa) for QLineEdit.