Fix cursors on some GNOME installations (#12914)

This PR adds support for `org.gnome.desktop.interface`'s `cursor-theme`
setting on Wayland. This should fix cursors not showing up on some GNOME
installs. This PR also adds the wiring to watch the current cursor theme
value.

Thanks to @apricotbucket28 for helping debug the issue.

Release Notes:

- N/A

crates/gpui/src/platform/linux/x11/client.rs

@@ -359,6 +359,9 @@ impl X11Client {
                             window.window.set_appearance(appearance);
                         }
                     }
+                    XDPEvent::CursorTheme(_) | XDPEvent::CursorSize(_) => {
+                        // noop, X11 manages this for us.
+                    }
                 }
             })
             .unwrap();