allow creation of subclasses of NodeItem for better draw customization

[coderbone]

I am currently subclassing Nodz to do some custom drawing and would like to use a subclass of NodeItem so that even more custom drawing can be done. To do this I propose splitting the line in createNode that actually instantiates a new NodeItem into it's own member function so that a subclass of Nodz can provide it's own implementation and return apropos subclass instances. Similar treatment would be applied Plug/Socket/Connection for full customization.

Any objections?

[predat]

👍

[coderbone]

I've put in PR #15