Allow the camerabin source selection to be overridden.

Prefer the default camera-source element if there is one or an element
identified by an environment variable to a static list of possible
elements which may not be appropriate for the target environment.

Change-Id: I53816c949307953780f9046eb11e09effe059be0
Reviewed-by: John Brooks <john.brooks@dereferenced.net>
Reviewed-by: Andy Nichols <andy.nichols@digia.com>
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>

src/plugins/gstreamer/camerabin/camerabinsession.h

@@ -117,6 +117,8 @@ public:
     QDir defaultDir(QCamera::CaptureModes mode) const;
     QString generateFileName(const QString &prefix, const QDir &dir, const QString &ext) const;
 
+    GstElement *buildCameraSource();
+
     CameraBinAudioEncoder *audioEncodeControl() const { return m_audioEncodeControl; }
     CameraBinVideoEncoder *videoEncodeControl() const { return m_videoEncodeControl; }
     CameraBinImageEncoder *imageEncodeControl() const { return m_imageEncodeControl; }
@@ -188,7 +190,6 @@ private slots:
 private:
     bool setupCameraBin();
     void setupCaptureResolution();
-    GstElement *buildCameraSource();
     static void updateBusyStatus(GObject *o, GParamSpec *p, gpointer d);
 
     QUrl m_sink;