DOCS/client_api_examples: add qt widget + opengl-cb example

DOCS/client_api_examples/qt_opengl/mainwindow.h

@@ -0,0 +1,27 @@
+#ifndef MainWindow_H
+#define MainWindow_H
+
+#include <QtWidgets/QWidget>
+
+class MpvWidget;
+class QSlider;
+class QPushButton;
+class MainWindow : public QWidget
+{
+    Q_OBJECT
+public:
+    explicit MainWindow(QWidget *parent = 0);
+public Q_SLOTS:
+    void openMedia();
+    void seek(int pos);
+    void pauseResume();
+private Q_SLOTS:
+    void setSliderRange(int duration);
+private:
+    MpvWidget *m_mpv;
+    QSlider *m_slider;
+    QPushButton *m_openBtn;
+    QPushButton *m_playBtn;
+};
+
+#endif // MainWindow_H