New trait type for pathlib.Path objects

Just like we have special trait types for `datetime` classes, there is a case to be made for having a special trait type for `pathlib.Path` instances above and beyond `Instance(Path)`:

- we want special editors to be available for paths
- we would like to be able to enforce additional constraints, such as the `Path` referring to a directory, or insisting that the file exist

This should go hand-in-hand with a corresponding editor or editors in TraitsUI.

The long term intent is that this should be a better replacement for `File` and `Directory` traits. See also #873.