videoPreview: Add Q_PROPERTY

Make it accessible to QML

Change-Id: Id3d88a1c56a48c8614d56ad80471a7833b1679e0
Reviewed-by: Stepan Salenikovich <stepan.salenikovich@savoirfairelinux.com>

src/video/previewmanager.h

@@ -35,6 +35,8 @@ class LIB_EXPORT PreviewManager : public QObject
    Q_OBJECT
 
 public:
+    Q_PROPERTY(bool previewing READ isPreviewing NOTIFY previewStateChanged)
+
    //Singleton
    static PreviewManager& instance();