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/auto_update/src/auto_update.rs

@@ -127,7 +127,7 @@ struct AutoUpdateSetting(bool);
 ///
 /// Default: true
 impl Settings for AutoUpdateSetting {
-    fn from_settings(content: &settings::SettingsContent, _cx: &mut App) -> Self {
+    fn from_settings(content: &settings::SettingsContent) -> Self {
         Self(content.auto_update.unwrap())
     }
 }