ClearAll does not clear Qualifiers

ClearAll  and Select.Clear does not clear qualifiers(First, Skip, Distinct).

Proposed change in GpSQLBuilder.AST.pas:

```
procedure TGpSQLSelect.Clear;
begin
  Columns.Clear;
  TableNames.Clear;
end;
```
to 
```
procedure TGpSQLSelect.Clear;
begin
  Columns.Clear;
  TableNames.Clear;
  Qualifiers.Clear;
end;
```

