MenuBar in a SubForm

[cursork]

MenuBar is listed as having both Form and SubForm as parents in the documentation. It works inside a SubForm when EWC is run in Browser mode, but not in Desktop mode. It does work in Browser mode for EWC, and both of these render very similarly:

  EWC.Init'Browser'

  'F'eWC'Form'
  'F.MB'eWC'MenuBar'
  'F.MB.X'eWC'Menu' 'xyz'
  'F.MB.X.Y'eWC'MenuItem' 'yyy'

  'F'eWC'Form'
  'F.SF'eWC'SubForm'
  'F.SF.MB'eWC'MenuBar'
  'F.SF.MB.X'eWC'Menu' 'xyz'
  'F.SF.MB.X.Y'eWC'MenuItem' 'yyy'

Trying with EWC.Init'Desktop' and a subform, the expression 'F.SF.MB'eWC'MenuBar' from above fails:

  ⍎VALUE ERROR: Undefined name: HR
  ∆WC[103] F.SF.HR
                ∧

Note that the first example above works in ⎕WC when the menu is on the form, but silently fails without error when the menu is on the subform:

  'F'⎕WC'Form'
  'F.SF'⎕WC'SubForm'
  'F.SF.MB'⎕WC'MenuBar'
  'F.SF.MB.X'⎕WC'Menu' 'A menu'
  'F.SF.MB.X.A'⎕WC'MenuItem' 'aaa'

The ⎕WC docs say that both Form and SubForm may be parents to MenuBar, but given that we do not currently have example applications using this and that it seems a basic example doesn't succeed in ⎕WC, I think we should improve the error case for adding a menubar to a subform.

Non-urgent, as I haven't seen this used in any potential production code.