Allow viewer co customize egui style for the individual node UI

[juh9870]

This would allow the viewer to tweak ui style of all node elements, instead of tweaking each one individually.

In my app, I use this to lock the node to light or dark mode, or edit their full style

[zakarumych]

Would it be possible to pass less than whole Ui into the method?
Note that taking &mut Style from Ui may require allocation, so should be avoided if no modifications are intended.

[juh9870]

Passing the whole UI is done precisely to avoid allocation. The default implementation of the method does nothing with the UI and doesn't allocate anything, but the downstream implementation may use this method to call ui.style_mut() to allocate and edit customs style.

An example in my app: https://github.com/juh9870/dbe/blob/382411ab25a7b676312c3e6613eeab74a786a5e3/dbe_ui/src/workspace/graph.rs#L104-L117

I use an external crate to modify the style of the passed UI.

[zakarumych]

I would like to avoid giving Ui so that user won't be able to modify anything but Style.
Passing impl DerefMut<Target = Style> comes to mind, but it looks heavy for the reader.

[juh9870]

You can also go the has_node_style and apply_node_style functions route

[zakarumych]

I don't like it, but it would be consistent with the rest of the API.

[juh9870]

Sorry for force pushes.

Changed to use has_node_style, apply_node_style

[zakarumych]

Last detail to change if you don't mind

[zakarumych]

Looks great, thanks!