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/JSIntegration.h

@@ -26,14 +26,12 @@
 
 #pragma once
 
+#include "Forward.h"
 #include <LibJS/Forward.h>
 #include <LibJS/Runtime/GlobalObject.h>
 
 namespace Spreadsheet {
 
-class Sheet;
-class Workbook;
-
 class SheetGlobalObject : public JS::GlobalObject {
     JS_OBJECT(SheetGlobalObject, JS::GlobalObject);