Add some demo examples.

src/app/ScribbleArea.h

@@ -0,0 +1,50 @@
+#ifndef SCRIBBLEAREA_H
+#define SCRIBBLEAREA_H
+
+#include <QColor>
+#include <QImage>
+#include <QPoint>
+#include <QWidget>
+
+//! [0]
+class ScribbleArea : public QWidget
+{
+    Q_OBJECT
+
+public:
+    ScribbleArea(QWidget *parent = nullptr);
+
+    bool openImage(const QString &fileName);
+    bool saveImage(const QString &fileName, const char *fileFormat);
+    void setPenColor(const QColor &newColor);
+    void setPenWidth(int newWidth);
+
+    bool isModified() const { return modified; }
+    QColor penColor() const { return myPenColor; }
+    int penWidth() const { return myPenWidth; }
+
+public slots:
+    void clearImage();
+    void print();
+
+protected:
+    void mousePressEvent(QMouseEvent *event) override;
+    void mouseMoveEvent(QMouseEvent *event) override;
+    void mouseReleaseEvent(QMouseEvent *event) override;
+    void paintEvent(QPaintEvent *event) override;
+    void resizeEvent(QResizeEvent *event) override;
+
+private:
+    void drawLineTo(const QPoint &endPoint);
+    void resizeImage(QImage *image, const QSize &newSize);
+
+    bool modified = false;
+    bool scribbling = false;
+    int myPenWidth = 1;
+    QColor myPenColor = Qt::blue;
+    QImage image;
+    QPoint lastPoint;
+};
+//! [0]
+
+#endif