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/CMakeLists.txt

@@ -1,4 +1,5 @@
 set(SOURCES
+    Cell.cpp
     CellSyntaxHighlighter.cpp
     HelpWindow.cpp
     JSIntegration.cpp