Plugins refactoring: handleReceived return enumeration ProcessMessage

Use `ProcessMessage::CONTINUE` to allows other modules to process a message.
Use `ProcessMessage::STOP` to stop further message processing.

src/plugins/TextMessagePlugin.h

@@ -17,9 +17,9 @@ class TextMessagePlugin : public SinglePortPlugin, public Observable<const MeshP
 
     /** Called to handle a particular incoming message
 
-    @return true if you've guaranteed you've handled this message and no other handlers should be considered for it
+    @return ProcessMessage::STOP if you've guaranteed you've handled this message and no other handlers should be considered for it
     */
-    virtual bool handleReceived(const MeshPacket &mp);
+    virtual ProcessMessage handleReceived(const MeshPacket &mp);
 };
 
 extern TextMessagePlugin *textMessagePlugin;