Improve handling of dates in cell editing UI

[wetneb]

Is your feature request related to a problem or area of OpenRefine? Please describe.

Say you want to manually edit a cell to input a date there. This is what it currently looks like:

The problems are:

• As a user, how do I know which formats are supported? Especially if I start with a text value and want to transform it to a date, I should not have to refer to the user manual to find out how to format it.
• When I try to input a wrong date, a JS alert pops up: "Not a valid date." That is not very helpful!

Describe the solution you'd like

Find a way to add a short description of the supported format(s) to the dialog itself, and/or improve the error message to mention the formats.

Describe alternatives you've considered

We could consider using an off-the-shelf date picker but it is not clear to me whether that would be really more user-friendly in a data cleaning context. Perhaps it could be used optionally, with still the ability to modify the date textually?

Additional context
Brought up in a documentation meeting with @allanaaa and @thadguidry.

[thadguidry]

Within that text input element... maybe it could just use ACE embeddable code editor with a bit of linting, and Syntax highlighting perhaps? (Other uses of ACE could be in our Expression Editor, but that's another issue)
addGutterDecoration
maybe others in the ACE API like setAnnotations in its virtual_renderer.

Scratch that... the Gutter Annotations are too subtle and the line numbering doesn't make sense for cell editing.