layout->add_constraint(*btn, GLtk::Layout::ALIGN_TOP, *buttons[0]);
layout->add_constraint(*btn, GLtk::Layout::ALIGN_BOTTOM, *buttons[1]);
layout->add_constraint(*btn, GLtk::Layout::ALIGN_LEFT, *buttons[3]);
layout->add_constraint(*btn, GLtk::Layout::ALIGN_TOP, *buttons[0]);
layout->add_constraint(*btn, GLtk::Layout::ALIGN_BOTTOM, *buttons[1]);
layout->add_constraint(*btn, GLtk::Layout::ALIGN_LEFT, *buttons[3]);