Update ImageGestures example

- Update screenshot
- Provide some information text on the screen when no images are found
- Add all supported image formats to the file filters
- Minor fixes according to our coding conventions: eg. don't shadow
  variables

Fixes: QTBUG-119979
Change-Id: If41adf34f38bfa101f2c5433082828c1a10668b1
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 30e6d822327d8105d209af10d077512f21ecc12c)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

examples/widgets/gestures/imagegestures/doc/src/imagegestures.qdoc

@@ -10,7 +10,7 @@
     This example shows how to enable gestures for a widget and use gesture input
     to perform actions.
 
-    \image imagegestures-example.jpg
+    \image imagegestures-example.png
 
     We use two classes to create the user interface for the application: \c MainWidget
     and \c ImageWidget. The \c MainWidget class is simply used as a container for the

examples/widgets/gestures/imagegestures/imagewidget.cpp

@@ -41,6 +41,12 @@ void ImageWidget::paintEvent(QPaintEvent*)
 {
     QPainter p(this);
 
+    if (files.isEmpty() && !path.isEmpty()) {
+        p.drawText(rect(), Qt::AlignCenter|Qt::TextWordWrap,
+                         tr("No supported image formats found"));
+        return;
+    }
+
     const qreal iw = currentImage.width();
     const qreal ih = currentImage.height();
     const qreal wh = height();
@@ -144,11 +150,15 @@ void ImageWidget::resizeEvent(QResizeEvent*)
     update();
 }
 
-void ImageWidget::openDirectory(const QString &path)
+void ImageWidget::openDirectory(const QString &url)
 {
-    this->path = path;
+    path = url;
     QDir dir(path);
-    const QStringList nameFilters{"*.jpg", "*.png"};
+
+    QStringList nameFilters;
+    const QList<QByteArray> supportedFormats = QImageReader::supportedImageFormats();
+    for (const QByteArray &format : supportedFormats)
+        nameFilters.append(QLatin1String("*.") + QString::fromLatin1(format));
     files = dir.entryInfoList(nameFilters, QDir::Files|QDir::Readable, QDir::Name);
 
     position = 0;

examples/widgets/gestures/imagegestures/imagewidget.h

@@ -25,7 +25,7 @@ class ImageWidget : public QWidget
 
 public:
     ImageWidget(QWidget *parent = nullptr);
-    void openDirectory(const QString &path);
+    void openDirectory(const QString &url);
     void grabGestures(const QList<Qt::GestureType> &gestures);
 
 protected: