LibIPC: Make sure we always process unhandled messages

A client that only ever does synchronous IPC calls from its side would
never actually process incoming asynchronous messages since they would
arrive while waiting for a synchronous response and then end up sitting
forever in the "unhandled messages" queue.

We now always handle unhandled messages using a deferred invocation.

This fixes the bug where Audio.MenuApplet didn't learn that the muted
state changed in response to its own request to change it. :^)

Libraries/LibIPC/IServerConnection.h

@@ -148,6 +148,12 @@ private:
             }
             ASSERT(decoded_bytes);
         }
+
+        if (!m_unprocessed_messages.is_empty()) {
+            deferred_invoke([this](auto&) {
+                handle_messages();
+            });
+        }
         return true;
     }