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/workspace/src/item.rs

@@ -65,7 +65,7 @@ pub struct PreviewTabsSettings {
 }
 
 impl Settings for ItemSettings {
-    fn from_settings(content: &settings::SettingsContent, _cx: &mut App) -> Self {
+    fn from_settings(content: &settings::SettingsContent) -> Self {
         let tabs = content.tabs.as_ref().unwrap();
         Self {
             git_status: tabs.git_status.unwrap(),
@@ -113,7 +113,7 @@ impl Settings for ItemSettings {
 }
 
 impl Settings for PreviewTabsSettings {
-    fn from_settings(content: &settings::SettingsContent, _cx: &mut App) -> Self {
+    fn from_settings(content: &settings::SettingsContent) -> Self {
         let preview_tabs = content.preview_tabs.as_ref().unwrap();
         Self {
             enabled: preview_tabs.enabled.unwrap(),