title_bar: Remove dependency on theme_selector (#21009)

This PR removes the `title_bar` crate's dependency on the
`theme_selector`.

The `theme_selector::Toggle` action now resides at
`zed_actions::theme_selector::Toggle`.

Release Notes:

- N/A

crates/zed/src/zed/app_menus.rs

@@ -23,7 +23,10 @@ pub fn app_menus() -> Vec<Menu> {
                             zed_actions::OpenDefaultKeymap,
                         ),
                         MenuItem::action("Open Project Settings", super::OpenProjectSettings),
-                        MenuItem::action("Select Theme...", theme_selector::Toggle::default()),
+                        MenuItem::action(
+                            "Select Theme...",
+                            zed_actions::theme_selector::Toggle::default(),
+                        ),
                     ],
                 }),
                 MenuItem::separator(),