Why does the Qt tutorial build the header class like this?

Question

I am learning how to use Qt. I am in the middle of the beginner's tutorials, and I ran into a problem.

I have this class here:

#ifndef WINDOW_H
#define WINDOW_H

#include <QWidget>

class QPushButton;
class Window : public QWidget
{
    public:
        explicit Window(QWidget *parent = 0);

    private:
        QPushButton *m_button;
};

#endif // WINDOW_H

I know the purpose of any row but this one:

class QPushButton;

In my all years I learned cpp, I never encountered this such a thing. I know that when I want to declare a class, I have to do this with this basic template:

class ClassName:
{
    public:
        //Optional public.
    private:
        //Optional private.
}

But one row declaration?

I want to ask more then that:

• Why do the Qt guys work like this?
• Why don't they #include" the QPushButton in the header?

They included this button class within the implementation. Why?:

#include "window.h"
#include <QPushButton>

Window::Window(QWidget *parent) :
 QWidget(parent)
{
    // Set size of the window
    setFixedSize(100, 50);

    // Create and position the button
    m_button = new QPushButton("Hello World", this);
    m_button->setGeometry(10, 10, 80, 30);
}