added scrollwheel support for GLFW

implemented scroll events based on the already provided scroll_cb in
ofAppGLFWWindow.cpp
usage in testApp:
void testApp::mouseScrolled(double x, double y){
    ofLog(OF_LOG_NOTICE, "scroll X:" + ofToString(x));
    ofLog(OF_LOG_NOTICE, "scroll Y:" + ofToString(y));
}