Add GtkTextIter Helpers

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Here is what is used in Denaro:

[StructLayout(LayoutKind.Sequential)]
public struct TextIter 
{
    public nint dummy1;
    public nint dummy2;
    public int dummy3;
    public int dummy4;
    public int dummy5;
    public int dummy6;
    public int dummy7;
    public int dummy8;
    public nint dummy9;
    public nint dummy10;
    public int dummy11;
    public int dummy12;
    public int dummy13;
    public nint dummy14;
}

[LibraryImport("libadwaita-1.so.0", StringMarshalling = StringMarshalling.Utf8)]
private static partial void gtk_text_buffer_get_bounds(nint buffer, ref TextIter startIter, ref TextIter endIter);

[LibraryImport("libadwaita-1.so.0", StringMarshalling = StringMarshalling.Utf8)]
private static partial string gtk_text_buffer_get_text(nint buffer, ref TextIter startIter, ref TextIter endIter, [MarshalAs(UnmanagedType.I1)]bool include_hidden_chars);