Add common trait for all frontends

[gbaranski]

closes #618

[archseer]

This runs into the discussions under #39, I was leaning towards either abstracting away the ui components, or at the very least sharing the components but keeping the rendering traits separate. That way frontends don't all have to look the same.

[dsseng]

Probably this has common tasks with client/server model. Backend is responsible of holding and operating on the code, frontend just tells what to do. They will then share a common protocol

[gbaranski]

I think I am not familiar enough with helix internals to implement that big change. It'd be better if someone else handle this.

@archseer

I was leaning towards either abstracting away the ui components, or at the very least sharing the components but keeping the rendering traits separate.

My idea was making Compositor trait, and all frontends would implement it, and it'd look something like that

pub trait Compositor {
    fn regex_prompt(
        &mut self,
        prompt: String,
        fun: Box<dyn FnOnce(&mut View, &mut Document, &mut Registers, Regex)>,
    );
    fn command_mode<'a>(
        &mut self,
        completion_fn: Box<dyn FnMut(&str) -> Vec<Completion> + 'static>,
        callback: Box<dyn FnOnce(&mut Context, &str, widgets::prompt::Event)>,
    );
    fn file_picker(&mut self, root: std::path::PathBuf);
    fn buffer_picker(&mut self);
    fn suspend(&mut self);
    // ...
}

[pickfire]

I don't think it is a good idea to couple file picker and the rest with Compositor.