Replace usage of Gtk.Menu with Gio.Menu #1045
Description
GtkMenu was removed in GTK4, which means we should start using menu models for our context menus. With menu models in place, we can keep using GtkMenu in GTK3, and easily switch to GtkPopoverMenu once we move to GTK4.
WIP branch from February, need to review and finish: https://github.com/mathiascode/nicotine-plus/commits/context-gmenu
Reference: https://gitlab.gnome.org/GNOME/gtk/-/blob/5710df685b0af9b7dd306dfba6c7e174e428950e/gtk/gtklabel.c#L5437-5515