Spreadsheet: Improve sheet update efficiency

There's no need to leave the cell dirty when not updating it, and
there's definitely no need to update the cells as we're selecting them.
This makes navigating a sheet and selecting cells significantly faster
as we no longer update unrelated cells just because they appear to have
a cyclic update dependency :^)

Applications/Spreadsheet/SpreadsheetModel.h

@@ -46,6 +46,7 @@ public:
     virtual void update() override;
     virtual bool is_column_sortable(int) const override { return false; }
     virtual StringView drag_data_type() const override { return "text/x-spreadsheet-data"; }
+    Sheet& sheet() { return *m_sheet; }
 
 private:
     explicit SheetModel(Sheet& sheet)