add menu support to the code generator

[matecsaj]

In the options part of the code generator, please add a drop-down for selecting the initial menu widget, if any.

When running the code generator on menu.ui, it would be helpful if the init dunder included the following line. self.mainwindow.configure(menu=self.builder.get_object('mainmenu', self.mainwindow))

After doing so, update the relevant wiki page and menu examples.

[alejandroautalan]

Completed in latest version.