Creating an editor to display a button opening a pop-up menu with actions

Hi there! This issue is to suggest the creation of a new editor, for convenience named `MenuButtonEditor` here, which would display a button that opens a menu when clicked, a bit like [this example](https://doc.qt.io/qt-5/qpushbutton.html#setMenu):


This editor could be the default one for a `Menu` trait for instance. It would otherwise work like a normal button, with the possibility to set its style (_normal_ or _toolbar_), its label and image. Ideally, it could be included in a toolbar.

A simple first pass could consist of using `QPushButton.setMenu()`, as it does exactly that (as shown in the picture above).

What do you think ?