Spreadsheet: Implement infinite-scroll for rows

Every time the scrollbar reaches the end, we append 100 more rows
(seamlessly!).
As a result of defaulting to 100 rows, we can also save with the
smallest number of rows required.
This partially deals with #4170.

Applications/Spreadsheet/SpreadsheetModel.cpp

@@ -170,6 +170,7 @@ void SheetModel::set_data(const GUI::ModelIndex& index, const GUI::Variant& valu
 void SheetModel::update()
 {
     m_sheet->update();
+    did_update(UpdateFlag::DontInvalidateIndexes);
 }
 
 }