Adds a widget to the list of mnemonic labels for this widget.
See [[email protected]_mnemonic_labels]. Note the list of mnemonic labels for the widget is cleared when the widget is destroyed, so the caller must make sure to update its internal state at this point as well.
| this |
a `GtkWidget` |
| label |
a `GtkWidget` that acts as a mnemonic label for this |