Use log::warn!() instead of println!()

This causes a functional change that may not be obvious.

 When running `cosmic-term` from a shell prompt as a detached
 process, `stdout`/`stderr` descriptors get deleted, and a `println!()`
 message would take the `cosmic-term` process down.

 No such an issue with log messages.

 This can be tested by running `htop` then exiting the tool, which
 would trigger a "TODO: CursorBlinkingChange" message.

Signed-off-by: Mohammad AlSaleh <CE.Mohammad.AlSaleh@gmail.com>

src/main.rs

@@ -878,7 +878,7 @@ impl Application for App {
                     }
                 }
                 _ => {
-                    println!("TODO: {:?}", event);
+                    log::warn!("TODO: {:?}", event);
                 }
             },
             Message::TermEventTx(term_event_tx) => {