DateEditor maximum and minimum name traits poorly implemented

The `DateEditor` factory allows specifying trait names to hold the earliest and latest dates that can be set. However they are implemented problematically:

- they aren't supported at all under Wx
- in the Qt backend they are evaluated once at control creation time, and not thereafter: there is no synchronization if the value of the names trait changes
- the first time they are evaluated, they modify the factory object to hold the values (this is perhaps bad design, rather than a bug, as each new editor will overwrite the values of the previous one)
- the function that gets the values is an `eval`-ed `lambda` rather than (a wrapper around) `xgetattr`