borrow instead of consume idgenerator

[TheNumenorean]

Why does the widget_ids! macros generate the struct to consume the id generator? It seems like it could be just borrowed, and that allows it to be reused for building multipl different id structs, which is good for compartmentalization.

[mitchmindtree]

This is fine with me if it will help to improve ergonomics 👍

Just thought I'd also mention that this probably won't affect performance at all as the widget ID generator is just a very thin wrapper around &mut Ui that acts kind of like an iterator.

One thing I've been considering is changing the generated struct to take a generic type that implements some IntoWidgetIdGenerator trait. This way we could allow the constructor function to take the generator directly or via &mut or even just &mut Ui. I'll likely implement this when I get around to implementing #1165 if noone beats me to it.

[TheNumenorean]

It would certainly be convenient, but waiting for the overhaul would be sufficient i suspect.

I would submit a pull req, but i am new to Rust and not confident enough yet.