Install WidgetsAskUserActionHandler header
Going to be used by widgets-based applications, e.g. Dolphin, to replace KIO::JobUiDelegate::askDeleteConfirmation by WidgetsAskUserActionHandler::askUserDelete().
WidgetsAskUserActionHandler is async, so no nested eventloop; the header wasn't installed because there were no use-cases for it, but while porting away from deprecated KIO::JobUiDelegate constructor, it's now clear that app code could used WidgetsAskUserActionHandler directly (just like it was using KIO::JobUiDelegate directly).