Move entities and managers back to Common (#431)

src/Spout/Writer/Common/Entity/Workbook.php

@@ -0,0 +1,50 @@
+<?php
+
+namespace Box\Spout\Writer\Common\Entity;
+
+/**
+ * Class Workbook
+ * Entity describing a workbook
+ *
+ * @package Box\Spout\Writer\Common\Entity
+ */
+class Workbook
+{
+    /** @var Worksheet[] List of the workbook's sheets */
+    private $worksheets = [];
+
+    /** @var string Timestamp based unique ID identifying the workbook */
+    private $internalId;
+
+    /**
+     * Workbook constructor.
+     */
+    public function __construct()
+    {
+        $this->internalId = uniqid();
+    }
+
+    /**
+     * @return Worksheet[]
+     */
+    public function getWorksheets()
+    {
+        return $this->worksheets;
+    }
+
+    /**
+     * @param Worksheet[] $worksheets
+     */
+    public function setWorksheets($worksheets)
+    {
+        $this->worksheets = $worksheets;
+    }
+
+    /**
+     * @return string
+     */
+    public function getInternalId()
+    {
+        return $this->internalId;
+    }
+}