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/audio/src/audio_settings.rs

@@ -42,7 +42,7 @@ pub struct AudioSettings {
 
 /// Configuration of audio in Zed
 impl Settings for AudioSettings {
-    fn from_settings(content: &settings::SettingsContent, _cx: &mut App) -> Self {
+    fn from_settings(content: &settings::SettingsContent) -> Self {
         let audio = &content.audio.as_ref().unwrap();
         AudioSettings {
             rodio_audio: audio.rodio_audio.unwrap(),