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/ui/src/components/list/list_sub_header.rs

@@ -1,10 +1,10 @@
 use crate::prelude::*;
-use crate::{h_stack, Icon, IconElement, IconSize, Label};
+use crate::{h_stack, Icon, IconName, IconSize, Label};
 
 #[derive(IntoElement)]
 pub struct ListSubHeader {
     label: SharedString,
-    start_slot: Option<Icon>,
+    start_slot: Option<IconName>,
     inset: bool,
 }
 
@@ -17,7 +17,7 @@ impl ListSubHeader {
         }
     }
 
-    pub fn left_icon(mut self, left_icon: Option<Icon>) -> Self {
+    pub fn left_icon(mut self, left_icon: Option<IconName>) -> Self {
         self.start_slot = left_icon;
         self
     }
@@ -40,11 +40,10 @@ impl RenderOnce for ListSubHeader {
                         .flex()
                         .gap_1()
                         .items_center()
-                        .children(self.start_slot.map(|i| {
-                            IconElement::new(i)
-                                .color(Color::Muted)
-                                .size(IconSize::Small)
-                        }))
+                        .children(
+                            self.start_slot
+                                .map(|i| Icon::new(i).color(Color::Muted).size(IconSize::Small)),
+                        )
                         .child(Label::new(self.label.clone()).color(Color::Muted)),
                 ),
         )