Add support for setting enable state of toolbar buttons

[brinkdinges]

When you call AddCommandItem2, you can pass a function to determine whether a toolbar button is enabled or disabled. It would be great if we add support for that. https://help.solidworks.com/2018/english/api/sldworksapi/solidworks.interop.sldworks~solidworks.interop.sldworks.icommandgroup~addcommanditem2.html

The place to add this would be in the CommandManagerGroup class, method AddCommandItem.