titleof the window, its
marginedattribute indicates that the window has a margin around its contents. It's a boolean attribute, so it doesn't need to have a value.
paddedattribute to the box container to add a space between its children. If you add the
horizontalattribute, the box layout will change from vertical to horizontal.
horizontalattribute, so the two buttons are displayed next to each other.
strechyattribute to a child widget to tell the Box container to increase the width (in a horizontal layout) or height (in a vertical layout) to fill the available space.
stretchyattribute, the available space is divided equally between them. For example, if you add this attribute to both buttons, each of them will expand to half of the window's width. When you resize the window, the buttons will be resized as well.