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/button/button.rs

@@ -2,7 +2,7 @@ use gpui::{AnyView, DefiniteLength};
 
 use crate::{prelude::*, IconPosition, KeyBinding};
 use crate::{
-    ButtonCommon, ButtonLike, ButtonSize, ButtonStyle, Icon, IconSize, Label, LineHeightStyle,
+    ButtonCommon, ButtonLike, ButtonSize, ButtonStyle, IconName, IconSize, Label, LineHeightStyle,
 };
 
 use super::button_icon::ButtonIcon;
@@ -14,11 +14,11 @@ pub struct Button {
     label_color: Option<Color>,
     label_size: Option<LabelSize>,
     selected_label: Option<SharedString>,
-    icon: Option<Icon>,
+    icon: Option<IconName>,
     icon_position: Option<IconPosition>,
     icon_size: Option<IconSize>,
     icon_color: Option<Color>,
-    selected_icon: Option<Icon>,
+    selected_icon: Option<IconName>,
     key_binding: Option<KeyBinding>,
 }
 
@@ -54,7 +54,7 @@ impl Button {
         self
     }
 
-    pub fn icon(mut self, icon: impl Into<Option<Icon>>) -> Self {
+    pub fn icon(mut self, icon: impl Into<Option<IconName>>) -> Self {
         self.icon = icon.into();
         self
     }
@@ -74,7 +74,7 @@ impl Button {
         self
     }
 
-    pub fn selected_icon(mut self, icon: impl Into<Option<Icon>>) -> Self {
+    pub fn selected_icon(mut self, icon: impl Into<Option<IconName>>) -> Self {
         self.selected_icon = icon.into();
         self
     }