U2: Add foam.u2.md.Checkbox for classical check boxes
U2: Add foam.u2.md.Checkbox for classical check boxes

jacksonic/foam,jlhughes/foam,jlhughes/foam,jacksonic/foam,foam-framework/foam,mdittmer/foam,mdittmer/foam,jacksonic/foam,jlhughes/foam,mdittmer/foam,foam-framework/foam,foam-framework/foam,jacksonic/foam,mdittmer/foam,foam-framework/foam,foam-framework/foam,jlhughes/foam