On X11, extract event handlers

Make code more clear wrt explicit returns during event handling,
which may lead to skipped IME event handling.

src/platform_impl/linux/mod.rs

@@ -940,10 +940,12 @@ impl EventLoopWindowTarget {
         }
     }
 
+    #[allow(dead_code)]
     fn set_exit_code(&self, code: i32) {
         x11_or_wayland!(match self; Self(evlp) => evlp.set_exit_code(code))
     }
 
+    #[allow(dead_code)]
     fn exit_code(&self) -> Option<i32> {
         x11_or_wayland!(match self; Self(evlp) => evlp.exit_code())
     }