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/diagnostics/src/toolbar_controls.rs

@@ -1,7 +1,7 @@
 use crate::ProjectDiagnosticsEditor;
 use gpui::{div, EventEmitter, ParentElement, Render, ViewContext, WeakView};
 use ui::prelude::*;
-use ui::{Icon, IconButton, Tooltip};
+use ui::{IconButton, IconName, Tooltip};
 use workspace::{item::ItemHandle, ToolbarItemEvent, ToolbarItemLocation, ToolbarItemView};
 
 pub struct ToolbarControls {
@@ -24,7 +24,7 @@ impl Render for ToolbarControls {
         };
 
         div().child(
-            IconButton::new("toggle-warnings", Icon::ExclamationTriangle)
+            IconButton::new("toggle-warnings", IconName::ExclamationTriangle)
                 .tooltip(move |cx| Tooltip::text(tooltip, cx))
                 .on_click(cx.listener(|this, _, cx| {
                     if let Some(editor) = this.editor.as_ref().and_then(|editor| editor.upgrade()) {