Add common trait for all frontends

[gbaranski]

that would unblock #39

[sudormrfbin]

I'm going to try to implement this, and the following is more or less the design I'm thinking of.

• Make Compositor a trait, which can be implemented by each frontend. It takes items which implement the Component trait and renders them as layers on layers.

• Move [structs implmenting] Components like FilePicker, EditorView, Popup, etc to helix-view. Now we have everything except the rendering logic in the base helix-view crate and frontends can handle rendering.

The Compositor and Component traits will be both defined in helix-view. The common "widgets" (items implementing Component) will also be in helix-view. These widgets will need to have the Component trait defined separately by each frontend. Since the frontend is another crate (like helix-term) we cannot do this:

// frontend crate
use helix_view::Component;
use helix_view::ui::FilePicker

// cannot implement a foreign trait for a foreign type
impl Component for FilePicker { /* ... */ }

We will need to use the newtype pattern and define wrapper structs for all widgets in frontends, which can then implement Component and also provide additional functionality if needed.

Any other ideas/suggestions welcome :)

[kirawi]

Closing in favor of #5555