Luca Bacci authored Oct 27, 2018 and Mike Fleetwood committed Nov 12, 2018

C++ initialises static member variables before main() is called.
Therefore the static members of:
    struct Slots
    {
        static Gtk::TreeModelColumn<Glib::ustring> text;
        static Gtk::TreeModelColumn<bool> sensitive;
    private:
        static Gtk::TreeModel::ColumnRecord record_;
    };
are constructed before Gtk::Main() is called in main().  However the
Gtkmm documentation specifically says that they must be constructed
afterwards [1].

Resolve this by using the Construct On First Use Idiom [2] to delay
initialisation until the slots are first used.  Normally this idiom uses
static local objects, however it is being applied to class static
objects here because the objects are accessed in many methods.  The
downside of this approach is that the objects are never destructed,
which memory analysers like Valgrind could see as a memory leak, but
that is actually deliberate.  That leak can be removed once we can use
C++11 and std::unique_ptr.

[1] gtkmm: Gtk::TreeModelColumnRecord Class Reference
    https://developer.gnome.org/gtkmm/2.24/classGtk_1_1TreeModelColumnRecord.html#details

    "Neither TreeModel::ColumnRecord nor the TreeModelColumns contain
    any real data - they merely describe what C++ type is stored in
    which column of a TreeModel, and save you from having to repeat that
    type information in several places.

    Thus TreeModel::ColumnRecord can be made a singleton (as long as you
    make sure it's instantiated after Gtk::Main), even when creating
    multiple models from it.
    "

[2] C++ FAQ / How do I prevent the "static initialization order
    problem"?
    https://isocpp.org/wiki/faq/ctors#static-init-order-on-first-use

Closes !17 - Gtk2 modernisation