Spreadsheet: Use new format functions.

In a few places I also simplified a few format strings:

    -outln("{} item{}", items, items.size() == 1 ? ' ' : 's');
    +outln("{} item(s)", items);

In my opinion this is more readable and in some places it incorrectly
wrote '0 item' which is "fixed" now. In other places the placeholder
space looked weird.

Applications/Spreadsheet/main.cpp

@@ -46,7 +46,7 @@ int main(int argc, char* argv[])
 
     if (filename) {
         if (!Core::File::exists(filename) || Core::File::is_directory(filename)) {
-            fprintf(stderr, "File does not exist or is a directory: %s\n", filename);
+            warnln("File does not exist or is a directory: {}", filename);
             return 1;
         }
     }