Gtk::Menu_Helpers::Element class and subclasses help in Gtk::MenuItem
widgets and also automate tasks like registering keyboard accelerators
when parented to a top-level window [1][2].
Gtk::Menu_Helpers::Element class and subclasses were removed in Gtkmm3
[3]. Provide compatible implementations under the namespace
GParted::Menu_Helpers.
References:
[1] gtkmm: Gtk::Menu_Helpers Namespace Reference
https://developer.gnome.org/gtkmm/2.24/namespaceGtk_1_1Menu__Helpers.html
[2] gtkmm: Gtk::Menu_Helpers::Element Class Reference
https://developer.gnome.org/gtkmm/2.24/classGtk_1_1Menu__Helpers_1_1Element.html
[3] Gtkmm 3 commit "MenuShell: Remove items()." removed the code
c8e47b0db5Closes#7 - Port to Gtk3