Remove cx from ThemeSettings (#38836)

Before this change the active theme and icon theme were retrofitted onto
the ThemeSettings.

Now they're in their own new global (GlobalTheme::theme(cx) and
GlobalTheme::icon_theme(cx))

This lets us remove cx from the settings traits, and tidy up a few other
things along the way.

Release Notes:

- N/A

crates/theme_extension/src/theme_extension.rs

@@ -5,7 +5,7 @@ use anyhow::Result;
 use extension::{ExtensionHostProxy, ExtensionThemeProxy};
 use fs::Fs;
 use gpui::{App, BackgroundExecutor, SharedString, Task};
-use theme::{ThemeRegistry, ThemeSettings};
+use theme::{GlobalTheme, ThemeRegistry};
 
 pub fn init(
     extension_host_proxy: Arc<ExtensionHostProxy>,
@@ -46,7 +46,7 @@ impl ExtensionThemeProxy for ThemeRegistryProxy {
     }
 
     fn reload_current_theme(&self, cx: &mut App) {
-        ThemeSettings::reload_current_theme(cx)
+        GlobalTheme::reload_theme(cx)
     }
 
     fn list_icon_theme_names(
@@ -83,6 +83,6 @@ impl ExtensionThemeProxy for ThemeRegistryProxy {
     }
 
     fn reload_current_icon_theme(&self, cx: &mut App) {
-        ThemeSettings::reload_current_icon_theme(cx)
+        GlobalTheme::reload_icon_theme(cx)
     }
 }