revert propert decoder of QZXingFilter that was mistakenly renamed. Allocate an instance of QZXing at the creation of QZXingFilter

src/QZXingFilter.cpp

@@ -28,6 +28,7 @@ namespace {
 QZXingFilter::QZXingFilter(QObject *parent)
     : QAbstractVideoFilter(parent)
     , decoding(false)
+    , decoder_p(new QZXing())
 {
     /// Conecting signals to handlers that will send signals to QML
     connect(decoder_p, &QZXing::decodingStarted,

src/QZXingFilter.h

@@ -63,7 +63,7 @@ class QZXingFilter : public QAbstractVideoFilter
 
     Q_OBJECT
         Q_PROPERTY(bool decoding READ isDecoding NOTIFY isDecodingChanged)
-        Q_PROPERTY(QZXing* decoder_p READ getDecoder)
+        Q_PROPERTY(QZXing* decoder READ getDecoder)
         Q_PROPERTY(QRectF captureRect MEMBER captureRect NOTIFY captureRectChanged)
 
     signals: