In DatetimeEditor, the dual role of "minimum_datetime" (default value _and_ enforced minimum) is limiting

When creating a UI with a datetime editor, one may want to provide the user with a useful default.

However, at present, the defalt value of the editor has to coincide with the enforced minimum value.

This leads to difficult compromises that may result in unreasonable constraints for the user (e.g. if developers choose the start of a year/decade as a "sensible" and round minimum/default, and the UI gets later used to attempt describing older dates).