`TableHeaderList<TItem, TOwner>` with string indexer

Create `TableHeaderList<TItem, TOwner>` class inherited from `ControlList<TItem, TOwner>` with indexer:

```cs
public TItem this[string headerText] { get; }
```

Change the property type of `Table<THeader, TRow, TOwner>.Headers` to the newly created type:

```cs
public TableHeaderList<THeader, TOwner> Headers { get; }
```