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/git_ui/src/text_diff_view.rs

@@ -450,7 +450,7 @@ mod tests {
     use gpui::{TestAppContext, VisualContext};
     use project::{FakeFs, Project};
     use serde_json::json;
-    use settings::{Settings, SettingsStore};
+    use settings::SettingsStore;
     use unindent::unindent;
     use util::{path, test::marked_text_ranges};
 
@@ -462,7 +462,7 @@ mod tests {
             Project::init_settings(cx);
             workspace::init_settings(cx);
             editor::init_settings(cx);
-            theme::ThemeSettings::register(cx)
+            theme::init(theme::LoadThemes::JustBase, cx);
         });
     }