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/language/src/language_settings.rs

@@ -500,7 +500,7 @@ fn merge_with_editorconfig(settings: &mut LanguageSettings, cfg: &EditorconfigPr
 }
 
 impl settings::Settings for AllLanguageSettings {
-    fn from_settings(content: &settings::SettingsContent, _cx: &mut App) -> Self {
+    fn from_settings(content: &settings::SettingsContent) -> Self {
         let all_languages = &content.project.all_languages;
 
         fn load_from_content(settings: LanguageSettingsContent) -> LanguageSettings {