code/models/EditableField.php
public function setSetting($key, $value)
    {
        $settings       = $this->getSettings();
        $settings[$key] = $value;

        $this->setSettings($settings);
    }
public function setSetting($key, $value)
    {
        $settings       = $this->getSettings();
        $settings[$key] = $value;

        $this->setSettings($settings);
    }
Set a given field setting. Appends the option to the settings or overrides
the existing value.

@param string $key
@param string $value