void gtk_file_chooser_native_set_cancel_label ( GtkFileChooserNative* self, const char* cancel_label )
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.