GLtk::Dropdown *drp = new GLtk::Dropdown(categories);
drp->signal_item_selected.connect(sigc::mem_fun(this, &DropdownDemo::category_selected));
GLtk::Dropdown *drp = new GLtk::Dropdown(categories);
drp->signal_item_selected.connect(sigc::mem_fun(this, &DropdownDemo::category_selected));