Allow editing entire date field at once (WIP)

[martinvanzijl]

Fixes #59.

Description

This adds an alternative date picker widget where you can edit the date manually all at once.

Demo

Here is a video demo:

(Drag and drop video here)

Known Issues

• I still have to test invalid dates.

Before merging the PR

• [ ] Add unit tests that cover any added or changed code.
• [ ] Update the help file.