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/image_viewer/src/image_viewer_settings.rs

@@ -1,4 +1,3 @@
-use gpui::App;
 pub use settings::ImageFileSizeUnit;
 use settings::Settings;
 
@@ -12,7 +11,7 @@ pub struct ImageViewerSettings {
 }
 
 impl Settings for ImageViewerSettings {
-    fn from_settings(content: &settings::SettingsContent, _cx: &mut App) -> Self {
+    fn from_settings(content: &settings::SettingsContent) -> Self {
         Self {
             unit: content.image_viewer.clone().unwrap().unit.unwrap(),
         }