Suggestion: Add a more general menu editor to Qt Designer

Qt Designer currently has a menu editor, but it only works on the menu bar of a QMainWindow. It does not seem to be possible to create an independent QMenu for use as a popup/context menu, and thus we must fall back on the programmer to create any menus that are not accessible from a QMainWindow's menu bar.

It would be useful to be able to create independent pop-up menus (and submenus) from within Designer. It would also be useful if it were possible to attach these stand-alone menus to the various objects that supported them, such as QPushButton, QToolButton, and QAction.