GUI: Clear messagefield after text has been processed

client/src/main/java/ch/zhaw/pm2/multichat/client/ChatWindowController.java

@@ -86,6 +86,7 @@ public class ChatWindowController {
             return;
         }
         String messageString = messageField.getText().strip();
+        messageField.clear();
         Matcher matcher = messagePattern.matcher(messageString);
         if (matcher.find()) {
             String receiver = matcher.group(1);