Ability to clear messages in 'queue'

[MrSam]

It can be useful to clear the queued messages:

    public function clearMessages() : void {
        if (isset($this->storage[$this->storageKey])) {
            $this->storage[$this->storageKey] = [];
        }
    }

    public function clearMessage(string $key) : void {
        if (isset($this->storage[$this->storageKey][$key])) {
            unset($this->storage[$this->storageKey][$key]);
        }
    }