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/settings/src/base_keymap_setting.rs

@@ -4,7 +4,6 @@ use crate::{
     self as settings,
     settings_content::{BaseKeymapContent, SettingsContent},
 };
-use gpui::App;
 use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
 use settings::{Settings, VsCodeSettings};
@@ -131,7 +130,7 @@ impl BaseKeymap {
 }
 
 impl Settings for BaseKeymap {
-    fn from_settings(s: &crate::settings_content::SettingsContent, _cx: &mut App) -> Self {
+    fn from_settings(s: &crate::settings_content::SettingsContent) -> Self {
         s.base_keymap.unwrap().into()
     }