`TableHeaderList<TItem, TOwner>` with string indexer

[YevgeniyShunevych]

Create TableHeaderList<TItem, TOwner> class inherited from ControlList<TItem, TOwner> with indexer:

public TItem this[string headerText] { get; }

Change the property type of Table<THeader, TRow, TOwner>.Headers to the newly created type:

public TableHeaderList<THeader, TOwner> Headers { get; }