Method app_menu_begin

libraries/lib-app-base/src/gl-imgui.cpp:664–668

    imgui_menu_stack::imgui_menu_stack(const polymer_app & app) : current_mods(app.get_mods()) { }

    void imgui_menu_stack::app_menu_begin()
    {
        assert(open.empty());
        open.push_back(ImGui::BeginMainMenuBar());
    }

    void imgui_menu_stack::begin(const char * label, bool enabled)
    {