Return the mnemonic accelerator.
If the label has been set so that it has a mnemonic key this function returns the keyval used for the mnemonic accelerator. If there is no mnemonic set up it returns `GDK_KEY_VoidSymbol`.
this |
a `GtkLabel` |
GDK keyval usable for accelerators, or `GDK_KEY_VoidSymbol` |