Enter and clear DateTime values manually

[tusmester]

DateTime fields should let the user enter and clear values manually, not only using the mouse.

• the control should be the usual textbox + icon combo
• help the user with default value (or placeholder text), to avoid confusion (month/date values!)
• entered time is local time, not UTC - but the control should post UTC
• QUESTION: what format do we accept? Local language?
  • 2023-08-09 09:05:47
  • currently displayed: 2023 August 09 05:47 p.m.
• accepted string should reflect field setting: date only, or date and time
• deleting the text: post null or min date?
• switch in CTD per field: old/new behavior

[tusmester]

This is an updated version of this older issue: sensenet/sensenet#1775