Move Sheet to Common/Entity (#438)

... and introduce a SheetManager

src/Spout/Writer/Common/Manager/WorkbookManagerInterface.php

@@ -3,7 +3,7 @@
 namespace Box\Spout\Writer\Common\Manager;
 
 use Box\Spout\Common\Exception\IOException;
-use Box\Spout\Writer\Common\Sheet;
+use Box\Spout\Writer\Common\Entity\Sheet;
 use Box\Spout\Writer\Common\Entity\Workbook;
 use Box\Spout\Writer\Common\Entity\Worksheet;
 use Box\Spout\Writer\Exception\SheetNotFoundException;