Enhanced date entry

[aDramaQueen]

Hi,
I added more convenience functionality to the DateEntry Widget:

• You can now easily enable/disable this Widget by simply calling the dedicated methods
• You can now change the date programmatically
• You can now provide/change a window title for the PopUp window of the calendar
• You can now provide a callback function, that is triggered automatically if the date has changed
• Added some type hints

---

NOTE:
As I understood, you are translating some of the predefined texts in this library? You may add the pre defined window title to the pool of these pre translated texts.

---

TODO:
You may considering adding some tests for the formater string. Right now you assume the given string represents a valid date formate. What happens if I give something like: %H:%M:%S

This is a perfectly valid formating string, but is absolutely useless for a date picker.

[Aareon]

Please add tests, otherwise LGTM