rename the dcc.DatePickerSingle `date` property to `value`

Is your feature request related to a problem? Please describe.
Pattern-matching callbacks are really helpful when collecting a large number of inputs in a callback, using the value component property. For example, using the following as callback inputs works really well:

Input({"type": "param", "index": ALL}, "id"),
Input({"type": "param", "index": ALL}, "value"),

But, this is complicated when a date from dcc.DatePickerSingle is a parameter, because the name of the property holding its 'value' is actually called date. It's workable to just pass something like Input("start-date", "date") as a separate callback input, but makes for messy code and to some extent defeats the elegance of pattern-matching.

Describe the solution you'd like
The component property from dcc.DatePickerSingle is called value instead of date.

Describe alternatives you've considered
The workaround mentioned above is the alternative I've implemented. You could also have a dcc.Input where the user types in a date, but then you'd lose the nice calendar UI element.

Start at the dcc.DatePickerSingle component entry point and trace where its date property is defined and exposed to callbacks. Check the related component and callback tests before assessing the compatibility impact; done means the property is consistently available as value without leaving the requested behavior ambiguous.