Consolidate UI infrastructure (fixes #441)

• start a design document explaining the widget system
• take factory methods out of Sidebar and UIShortcut, so that later they can be split into packages
• redesign Sidebar arguments:
  • icon
  • label (description of what the field is about)
  • raw (that becomes cooked if there is no cooked, or the tooltip if there is) (i.e. "2024-07-06 12:32:12")
  • cooked the presentation of the value to the user (i.e. "5 days ago")