Group.InitChange -- prepare group for addition or removal of children (V1.2)
Prepares a group for dynamic adding/removing of objects. Since version 3 MUI
offers the possibility to dynamically add/remove children from groups, even
when the window that contains these objects is currently open. To be able to do this,
you must first put the group into a special "exchange" state by using this method.
Then, you can add/remove children at will. If you're done, use Group.ExitChange
to make MUI recalculate the display.