highgui: add property WND_PROP_VISIBLE

checks whether the window exists and is visible. On QT closing a window
merley hides it, so the common hack for checking whether a window exists

exists = cv2.getWindowProperty(.., 0) >= 0

does not work.

modules/highgui/include/opencv2/highgui.hpp

@@ -196,7 +196,8 @@ enum WindowPropertyFlags {
        WND_PROP_FULLSCREEN   = 0, //!< fullscreen property    (can be WINDOW_NORMAL or WINDOW_FULLSCREEN).
        WND_PROP_AUTOSIZE     = 1, //!< autosize property      (can be WINDOW_NORMAL or WINDOW_AUTOSIZE).
        WND_PROP_ASPECT_RATIO = 2, //!< window's aspect ration (can be set to WINDOW_FREERATIO or WINDOW_KEEPRATIO).
-       WND_PROP_OPENGL       = 3  //!< opengl support.
+       WND_PROP_OPENGL       = 3, //!< opengl support.
+       WND_PROP_VISIBLE      = 4  //!< checks whether the window exists and is visible
      };
 
 //! Mouse Events see cv::MouseCallback

modules/highgui/include/opencv2/highgui/highgui_c.h

@@ -111,6 +111,7 @@ enum
     CV_WND_PROP_AUTOSIZE   = 1, //to change/get window's autosize property
     CV_WND_PROP_ASPECTRATIO= 2, //to change/get window's aspectratio property
     CV_WND_PROP_OPENGL     = 3, //to change/get window's opengl support
+    CV_WND_PROP_VISIBLE    = 4,
 
     //These 2 flags are used by cvNamedWindow and cvSet/GetWindowProperty
     CV_WINDOW_NORMAL       = 0x00000000, //the user can resize the window (no constraint)  / also use to switch a fullscreen window to a normal size