Added document about Qt framework (#1444)

* Added document about Qt framework

* Removed qt.cpp file and made some corrections in the markdown file

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+---
+category: tool
+tool: Qt Framework
+language: c++
+filename: learnqt.cpp
+contributors:
+    - ["Aleksey Kholovchuk", "https://github.com/vortexxx192"]
+lang: en
+---
+
+**Qt** is a widely-known framework for developing cross-platform software that can be run on various software and hardware platforms with little or no change in the code, while having the power and speed of native applications. Though **Qt** was originally written in *C++*, there are its ports to other languages: *PyQt*, *QtRuby*, *PHP-Qt*, etc.
+
+**Qt** is beautiful for creating applications with graphical user interface (GUI). This tutorial is how to do it in *C++*.
+
+```c++
+/*
+ * Let's start clasically
+ */
+
+// all headers from Qt framework start with capital letter 'Q'
+#include <QApplication>
+#include <QLineEdit>
+
+int main(int argc, char *argv[]) {
+	 // create an object to manage application-wide resources
+    QApplication app(argc, argv);
+
+    // create line edit widget and show it on screen
+    QLineEdit lineEdit("Hello world!");
+    lineEdit.show();
+
+    // start the application's event loop
+    return app.exec();
+}
+```
+
+GUI-related part of **Qt** is all about *widgets* and *connections* between them.
+
+[READ MORE ABOUT WIDGETS](http://doc.qt.io/qt-5/qtwidgets-index.html)
+
+```c++
+/*
+ * Let's create a label and a button.
+ * A label should appear when a button is pressed.
+ * 
+ * Qt code is speaking for itself.
+ */
+ 
+#include <QApplication>
+#include <QDialog>
+#include <QVBoxLayout>
+#include <QPushButton>
+#include <QLabel>
+
+int main(int argc, char *argv[]) {
+    QApplication app(argc, argv);
+
+    QDialog dialogWindow;
+    dialogWindow.show();
+    
+    // add vertical layout 
+    QVBoxLayout layout;
+    dialogWindow.setLayout(&layout);  
+
+    QLabel textLabel("Thanks for pressing that button");
+    layout.addWidget(&textLabel);
+    textLabel.hide();
+
+    QPushButton button("Press me");
+    layout.addWidget(&button);
+    
+    // show hidden label when the button is pressed
+    QObject::connect(&button, &QPushButton::pressed,
+                     &textLabel, &QLabel::show);
+
+    return app.exec();
+}
+```
+
+Notice that *QObject::connect* part. This method is used to connect *SIGNALS* of one objects to *SLOTS* of another.
+
+**Signals** are being emited when certain things happen with objects, like *pressed* signal is emited when user presses on QPushButton object.
+
+**Slots** are *actions* that might be performed in response to received signals.
+
+[READ MORE ABOUT SLOTS AND SIGNALS](http://doc.qt.io/qt-4.8/signalsandslots.html)
+
+
+Next, let's learn that we can not only use standard widgets but also extend their behaviour using inheritance. Let's create a button and count how many times it was pressed. For this purpose we define our own class *CounterLabel*.  It must be declared in separate file because of specific Qt architecture.
+
+```c++
+// counterlabel.hpp
+
+#ifndef COUNTERLABEL
+#define COUNTERLABEL
+
+#include <QLabel>
+
+class CounterLabel : public QLabel {
+    Q_OBJECT  // Qt-defined macros that must be present in every custom widget
+
+public:
+    CounterLabel() : counter(0) {
+        setText("Counter has not been increased yet");  // method of QLabel
+    }
+
+public slots:
+    // action that will be called in response to button press
+    void increaseCounter() {
+        setText(QString("Counter value: %1").arg(QString::number(++counter)));
+    }
+
+private:
+    int counter;
+};
+
+#endif // COUNTERLABEL
+```
+
+```c++
+// main.cpp
+// Almost the same as in previous example
+
+#include <QApplication>
+#include <QDialog>
+#include <QVBoxLayout>
+#include <QPushButton>
+#include <QString>
+#include "counterlabel.hpp"
+
+int main(int argc, char *argv[]) {
+    QApplication app(argc, argv);
+
+    QDialog dialogWindow;
+    dialogWindow.show();
+
+    QVBoxLayout layout;
+    dialogWindow.setLayout(&layout);
+
+    CounterLabel counterLabel;
+    layout.addWidget(&counterLabel);
+
+    QPushButton button("Push me once more");
+    layout.addWidget(&button);
+    QObject::connect(&button, &QPushButton::pressed,
+                     &counterLabel, &CounterLabel::increaseCounter);
+
+    return app.exec();
+}
+```
+
+## Furter reading
+That's it! Of course Qt framework is much much larger than the part that was covered in this tutorial, so be ready to read and practice.
+
+[READ MORE ABOUT QT](http://doc.qt.io/qt-4.8/tutorials.html)
+
+Good luck and have fun