fix: sysManager.LookupTreeItem is not working with the latest 4026 beta

[iadonkey]

todo:

• create and use property for the library manager
• use the property instead of code duplication, there are at least 3 more places where the libManager is looked up like this

[iadonkey]

I think we should ask Beckhoff if this is intendet or not ... ITcSysManager16::LookupTreeItem behaves differently as before since the latest beta

var plc = _plc.Object as dynamic;

// Code that was working in the previous beta and TC3.1.4024.xx
// var sysManager = plc.SystemManager as ITcSysManager2;
// var libManager = sysManager.LookupTreeItem(plc.PathName + "^References") as ITcPlcLibraryManager;

// Now we have todo
var libManagerItem = plc.LookupChild("References");
var libManager = libManagerItem as ITcPlcLibraryManager;

[iadonkey]

already merged fix with #120