Sets the custom label text for the cancel button.
If characters in label are preceded by an underscore, they are underlined. If you need a literal underscore character in a
label, use “__” (two underscores). The first underlined character represents a keyboard accelerator called a mnemonic.
Pressing Alt and that key should activate the button.
| this |
a `GtkFileChooserNative` |
| cancel_label |
custom label |