Supporting usage of NodeConstructor's contained in a struct

[theoparis]

See https://github.com/tinted-software/egui_nodes/blob/main/examples/simple.rs#L12

I want to be able to use NodeConstructor's from a struct like MyApp so that the nodes can be created upon startup and updated later on instead of during rendering however I ran into mutable reference issues and I'm not sure how to proceed...

[theoparis]

Hmm, It seems like https://github.com/zakarumych/egui-snarl is a better crate overall. It looks better visually as well. I might end up deprecating this crate and maybe PR'ing additional features like snap to grid there instead.