(core) Fix some bugs with repositioning rows.

Summary:
- Fixed an issue with manualSort values being very close floats. It is already handled by the data engine, but the client was being unnecessarily proactive and introduced a bug.
- The fix also helps with rearranging rows in filtered situations: they will now stay next to the row before which they were inserted.
- The fix accidentally improves (though doesn't fully fix) the issue where new columns show up in unexpected places in the raw-data column list.
- Fixed another rare bug with row order not getting updated correctly when positions update.

Test Plan: Added test cases for the improved behavior; fixed affected tests.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3462

app/client/ui/VisibleFieldsConfig.ts

@@ -401,7 +401,7 @@ export class VisibleFieldsConfig extends Disposable {
 }
 
 function getFieldNewPosition(fields: KoArray<ViewFieldRec>, item: IField,
-                             nextField: ViewFieldRec|null): number {
+                             nextField: ViewFieldRec|null): number|null {
   const index = getItemIndex(fields, nextField);
   return tableUtil.fieldInsertPositions(fields, index, 1)[0];
 }