insert
Description:
Insert the child into the this at position.
If a sort function is set, the widget will actually be inserted at the calculated position and this function has the same effect of add.
If position is -1, or larger than the total number of items in the this, then the child
       will be appended to the end.
Parameters:
| this | 
           a ListBox  | 
      
| child | 
           the Widget to add  | 
      
| position | 
           the position to insert   |