Insert the given attribute into the `PangoAttrList`.
It will replace any attributes of the same type on that segment and be merged with any adjoining attributes that are identical.
This function is slower than [[email protected]] for creating an attribute list in order (potentially much slower for large lists). However, [[email protected]] is not suitable for continually changing a set of attributes since it never removes or combines existing attributes.
| this |
a `PangoAttrList` |
| attr |
the attribute to insert |