Spreadsheet: Implement undo functionality where missing

Implement undo/redo functionality in the Spreadsheet application
for the "extend" function, the drag-and-drop function, and when
copying and pasting.

Userland/Applications/Spreadsheet/SpreadsheetModel.h

@@ -30,6 +30,7 @@ public:
     void update();
 
     Function<void(Cell&, String&)> on_cell_data_change;
+    Function<void(Vector<CellChange>)> on_cells_data_change;
 
 private:
     explicit SheetModel(Sheet& sheet)