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/dap/src/debugger_settings.rs

@@ -1,5 +1,4 @@
 use dap_types::SteppingGranularity;
-use gpui::App;
 use settings::{Settings, SettingsContent};
 
 pub struct DebuggerSettings {
@@ -34,7 +33,7 @@ pub struct DebuggerSettings {
 }
 
 impl Settings for DebuggerSettings {
-    fn from_settings(content: &SettingsContent, _cx: &mut App) -> Self {
+    fn from_settings(content: &SettingsContent) -> Self {
         let content = content.debugger.clone().unwrap();
         Self {
             stepping_granularity: dap_granularity_from_settings(