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/git_panel_settings.rs

@@ -43,7 +43,7 @@ impl ScrollbarVisibility for GitPanelSettings {
 }
 
 impl Settings for GitPanelSettings {
-    fn from_settings(content: &settings::SettingsContent, _cx: &mut ui::App) -> Self {
+    fn from_settings(content: &settings::SettingsContent) -> Self {
         let git_panel = content.git_panel.clone().unwrap();
         Self {
             button: git_panel.button.unwrap(),