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/recent_projects/src/remote_connections.rs

@@ -104,7 +104,7 @@ impl From<WslConnection> for Connection {
 }
 
 impl Settings for SshSettings {
-    fn from_settings(content: &settings::SettingsContent, _cx: &mut App) -> Self {
+    fn from_settings(content: &settings::SettingsContent) -> Self {
         let remote = &content.remote;
         Self {
             ssh_connections: remote.ssh_connections.clone().unwrap_or_default().into(),