Rename IconElement to just Icon (#3974)

This PR renames the `IconElement` component to just `Icon`.

This better matches the rest of our components, as `IconElement` was the
only one using this naming convention.

The `Icon` enum has been renamed to `IconName` to free up the name.

I was trying to come up with a way that would allow rendering an
`Icon::Zed` directly (and thus make the `IconElement` a hidden part of
the API), but I couldn't come up with a way to do this cleanly.

Release Notes:

- N/A

crates/workspace/src/shared_screen.rs

@@ -12,7 +12,7 @@ use gpui::{
     WindowContext,
 };
 use std::sync::{Arc, Weak};
-use ui::{h_stack, prelude::*, Icon, IconElement, Label};
+use ui::{h_stack, prelude::*, Icon, IconName, Label};
 
 pub enum Event {
     Close,
@@ -100,7 +100,7 @@ impl Item for SharedScreen {
     ) -> gpui::AnyElement {
         h_stack()
             .gap_1()
-            .child(IconElement::new(Icon::Screen))
+            .child(Icon::new(IconName::Screen))
             .child(
                 Label::new(format!("{}'s screen", self.user.github_login)).color(if selected {
                     Color::Default