[WidgetsAskUserActionHandler] Add fallbacks for dialog parent windows
requestUserMessageBox may be called without a parent widget, e.g. because it is called from KIOCore where we don't have that information
Apply the same logic we have for other functions
This ensures that message boxes triggered from workers have a parent window
Set window when creating delegate. The ctor introduced in 3f0ca7a6 missed this, only the old ctor had this
Edited by Nicolas Fella