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/vim_mode_setting/src/vim_mode_setting.rs

@@ -16,7 +16,7 @@ pub fn init(cx: &mut App) {
 pub struct VimModeSetting(pub bool);
 
 impl Settings for VimModeSetting {
-    fn from_settings(content: &SettingsContent, _cx: &mut App) -> Self {
+    fn from_settings(content: &SettingsContent) -> Self {
         Self(content.vim_mode.unwrap())
     }
 
@@ -28,7 +28,7 @@ impl Settings for VimModeSetting {
 pub struct HelixModeSetting(pub bool);
 
 impl Settings for HelixModeSetting {
-    fn from_settings(content: &SettingsContent, _cx: &mut App) -> Self {
+    fn from_settings(content: &SettingsContent) -> Self {
         Self(content.helix_mode.unwrap())
     }