Split into model library and view library

[kkngsm]

I have an idea to split the current egui-node-graph into a model library (like node-graph-core) and a GUI library (like node-graph-egui).

The benefits of this are twofold.

1. It will be easier to support libraries other than egui.
2. Allows implementation of heavily customized UIs such as Generalizing the diagram capabilities #48 2. Custom symbol rendering

The disadvantage is that this is too big a change to take time and effort, and the API changes significantly, but it may be relatively easy while the library is new.

[setzer22]

I think trying to generalize the data structure of this library so that it can be exposed to other GUI frameworks falls a bit out of of scope. It's likely that changes that are good for one framework are detrimental to others, and my general experience is that the more "cross-framework" a library becomes, the more tradeoffs one needs to take into account.

That said, the ideas discussed in 2 are currently being implemented by @mxgrey, and some of the changes in that branch already go in the direction you're asking, like the ability to provide fully-customizable nodes or ports 😄 #30 (comment)

[mxgrey]

I was considering separating out the graph logic from the GUI details while I worked on the changes that I'm making, but decided that it would be better to do that incrementally, if at all. I.e. make it fully generalized for egui and then only after that's settled consider if it can be refactored further to be agnostic to the renderer.