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/search/src/search.rs

@@ -60,11 +60,11 @@ impl SearchOptions {
         }
     }
 
-    pub fn icon(&self) -> ui::Icon {
+    pub fn icon(&self) -> ui::IconName {
         match *self {
-            SearchOptions::WHOLE_WORD => ui::Icon::WholeWord,
-            SearchOptions::CASE_SENSITIVE => ui::Icon::CaseSensitive,
-            SearchOptions::INCLUDE_IGNORED => ui::Icon::FileGit,
+            SearchOptions::WHOLE_WORD => ui::IconName::WholeWord,
+            SearchOptions::CASE_SENSITIVE => ui::IconName::CaseSensitive,
+            SearchOptions::INCLUDE_IGNORED => ui::IconName::FileGit,
             _ => panic!("{:?} is not a named SearchOption", self),
         }
     }