OfxGui: utf-8 and reacting on ofxButton press?

I am using:

$ (cd openFrameworks/; git log -1 --decorate | head -3)
commit 16eca45a1d45a6a98621756cabad0e2a9b85ff44 (HEAD, origin/master, origin/HEAD, master)
Author: arturo castro <arturo@openframeworks.cc>
Date:   Sat Sep 19 12:18:44 2015 +0200
$ cat /etc/issue
Ubuntu 14.04.3 LTS \n \l

I have prepared the following example, first by running the command line project generator:

openFrameworks/apps/projectGenerator/commandLine/bin/projectGenerator \
-a"ofxGui" -o"openFrameworks" \
openFrameworks/examples/addons/guiBtnTest

... and then replacing the ofApp.{cpp,h} in examples/addons/guiBtnTest/src with these files:

ofApp.h:

#pragma once

#include "ofMain.h"
#include "ofxGui.h" //addon here too

class ofApp : public ofBaseApp{

  public:
    void setup();
    void update();
    void draw();

    ofxButton testButton;
    void testButtonChange(bool & inval);
    ofxToggle testToggle;
    void testToggleChange(bool & inval);
    ofParameter<bool> testBoolParam;
    void testBoolParamChange(bool & inval);
    ofxPanel gui;

    void keyPressed(int key);
    void keyReleased(int key);
    void mouseMoved(int x, int y );
    void mouseDragged(int x, int y, int button);
    void mousePressed(int x, int y, int button);
    void mouseReleased(int x, int y, int button);
    void mouseEntered(int x, int y);
    void mouseExited(int x, int y);
    void windowResized(int w, int h);
    void dragEvent(ofDragInfo dragInfo);
    void gotMessage(ofMessage msg);

};

ofApp.cpp:

#include "ofApp.h"

//--------------------------------------------------------------
void ofApp::setup(){

  ofSetVerticalSync(true);
  ofSetLogLevel(OF_LOG_NOTICE);

  // this **** line causes:
  /*
ofEvent.h:407:93: error: no matching function for call to ‘ofEvent<void>::make_function(ofApp*&, void (ofApp::*&)(bool&), int&)’
   ofBaseEvent<of::priv::Function<void>,Mutex>::remove(make_function(listener,method,priority));
ofEvent.h:402:90: note: candidates are:
...
  */
  //testButton.addListener(this, &ofApp::testButtonChange); //****

  testToggle.addListener(this, &ofApp::testToggleChange);
  testBoolParam.addListener(this, &ofApp::testBoolParamChange);

  gui.setup("my panel"); // most of the time you don't need a name but don't forget to call setup
  gui.add(testButton.setup("testButton ← !"));
  gui.add(testToggle.setup("testToggle ← !", true));
  gui.add(testBoolParam.set("testBoolParam ← !", true));
}

//--------------------------------------------------------------
void ofApp::testButtonChange(bool & inval){
  if (inval) {
    ofLogNotice() << "testButtonChange: true" << endl;
  } else {
    ofLogNotice() << "testButtonChange: false" << endl;
  }
}

//--------------------------------------------------------------
void ofApp::testToggleChange(bool & inval){
  if (inval) {
    ofLogNotice() << "testToggleChange: true" << endl;
  } else {
    ofLogNotice() << "testToggleChange: false" << endl;
  }
}

//--------------------------------------------------------------
void ofApp::testBoolParamChange(bool & inval){
  if (inval) {
    ofLogNotice() << "testBoolParamChange: true" << endl;
  } else {
    ofLogNotice() << "testBoolParamChange: false" << endl;
  }
}

//--------------------------------------------------------------
void ofApp::update(){

}

//--------------------------------------------------------------
void ofApp::draw(){
  ofBackgroundGradient(ofColor::white, ofColor::gray);
  gui.draw();
}

void ofApp::keyPressed(int key){}
void ofApp::keyReleased(int key){}
void ofApp::mouseMoved(int x, int y ){}
void ofApp::mouseDragged(int x, int y, int button){}
void ofApp::mousePressed(int x, int y, int button){}
void ofApp::mouseReleased(int x, int y, int button){}
void ofApp::mouseEntered(int x, int y){}
void ofApp::mouseExited(int x, int y){}
void ofApp::windowResized(int w, int h){}
void ofApp::gotMessage(ofMessage msg){}
void ofApp::dragEvent(ofDragInfo dragInfo){}

Then I build and run with:

cd openFrameworks/examples/addons/guiBtnTest/
make -j 2 Release
MESA_GL_VERSION_OVERRIDE=2.1 ./bin/guiBtnTest

The application looks something like this:

As you can see, I can relatively easily get reaction on clicks from ofxToggle and ofParameter<bool>, simply by calling addListener. However, the same is not valid for ofxButton - as soon as I try to addListener to it - I get compilation errors of the error: no matching function for call type (try uncommenting the **** line in ofApp.cpp).

So, my questions are:

• Is it possible somehow to receive an event/trigger on press and release of ofxButton in ofxGui? If so, how?
• The text used in the app is "test* ← !"; as you can see, there is an utf-8 arrow '←' in there, however, it is not rendered in the app. Is it possible to get extended utf-8 characters in ofxGui widgets - and if so, how?

Thanks in advance for any answers,
Cheers!