Regarding XUL layout management

　　I think it not easy to arrange my elements in the xul container.Layout management can only be achieved by vbox and hbox.

　　Still there are some useful properties that may be useful.

• pack: the pack attribute has only three optional values:start,center and end.Buy setting it to start,all the elements in the container will be left or top aligned,in the contrast with setting it to end.
• flex:Indicates the flexibility of the element, which indicates how an element's container distributes remaining empty space among its children. Flexible elements grow and shrink to fit their given space. Elements with larger flex values will be made larger than elements with lower flex values, at the ratio determined by the two elements. The actual value is not relevant unless there are other flexible elements within the same container. Once the default sizes of elements in a box are calculated, the remaining space in the box is divided among the flexible elements, according to their flex ratios. Specifying a flex value of 0 has the same effect as leaving the flex attribute out entirely.