insert
Description:
Insert the given attribute into the `PangoAttrList`.
It will be inserted after all other attributes with a matching start_index.
Parameters:
| this | 
           a `PangoAttrList`  | 
      
| attr | 
           the attribute to insert  | 
      
Insert the given attribute into the `PangoAttrList`.
It will be inserted after all other attributes with a matching start_index.
| this | 
           a `PangoAttrList`  | 
      
| attr | 
           the attribute to insert  |