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/dock.rs

@@ -28,7 +28,7 @@ pub trait Panel: FocusableView + EventEmitter<PanelEvent> {
     fn set_position(&mut self, position: DockPosition, cx: &mut ViewContext<Self>);
     fn size(&self, cx: &WindowContext) -> Pixels;
     fn set_size(&mut self, size: Option<Pixels>, cx: &mut ViewContext<Self>);
-    fn icon(&self, cx: &WindowContext) -> Option<ui::Icon>;
+    fn icon(&self, cx: &WindowContext) -> Option<ui::IconName>;
     fn icon_tooltip(&self, cx: &WindowContext) -> Option<&'static str>;
     fn toggle_action(&self) -> Box<dyn Action>;
     fn icon_label(&self, _: &WindowContext) -> Option<String> {
@@ -52,7 +52,7 @@ pub trait PanelHandle: Send + Sync {
     fn set_active(&self, active: bool, cx: &mut WindowContext);
     fn size(&self, cx: &WindowContext) -> Pixels;
     fn set_size(&self, size: Option<Pixels>, cx: &mut WindowContext);
-    fn icon(&self, cx: &WindowContext) -> Option<ui::Icon>;
+    fn icon(&self, cx: &WindowContext) -> Option<ui::IconName>;
     fn icon_tooltip(&self, cx: &WindowContext) -> Option<&'static str>;
     fn toggle_action(&self, cx: &WindowContext) -> Box<dyn Action>;
     fn icon_label(&self, cx: &WindowContext) -> Option<String>;
@@ -104,7 +104,7 @@ where
         self.update(cx, |this, cx| this.set_size(size, cx))
     }
 
-    fn icon(&self, cx: &WindowContext) -> Option<ui::Icon> {
+    fn icon(&self, cx: &WindowContext) -> Option<ui::IconName> {
         self.read(cx).icon(cx)
     }
 
@@ -774,7 +774,7 @@ pub mod test {
             self.size = size.unwrap_or(px(300.));
         }
 
-        fn icon(&self, _: &WindowContext) -> Option<ui::Icon> {
+        fn icon(&self, _: &WindowContext) -> Option<ui::IconName> {
             None
         }