Luca Bacci authored Aug 02, 2018 and Mike Fleetwood committed Feb 11, 2019

GParted uses Gtk::Menu_Helpers::MenuList helper class to access
individual menu items.  This helper class made it easy to navigate menu
items by index.

Gtk::Menu_Helpers::MenuList was removed in the switch from Gtkmm2 to
Gtkmm3 [1].  Instead, use a separate std::map<Gtk::MenuItem*> to keep
track of individual Gtk::MenuItem objects.

Reference:
[1] Gtkmm 3 commit "MenuShell: Remove items()." removed the code
    gtkmm@c8e47b0d

Closes #7 - Port to Gtk3