Spreadsheet: Reorganise the sources a bit

This commit just moves some code around:
- Give Cell its own file
- Pull all forward-declared classes/structs into Forward.h
- Clean up the order of member functions a bit

Applications/Spreadsheet/Workbook.h

@@ -26,14 +26,13 @@
 
 #pragma once
 
+#include "Forward.h"
 #include "Spreadsheet.h"
 #include <AK/NonnullOwnPtrVector.h>
 #include <AK/Result.h>
 
 namespace Spreadsheet {
 
-class WorkbookObject;
-
 class Workbook {
 public:
     Workbook(NonnullRefPtrVector<Sheet>&& sheets);