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/src/window_QT.cpp

@@ -138,6 +138,20 @@ double cvGetRatioWindow_QT(const char* name)
     return result;
 }
 
+double cvGetPropVisible_QT(const char* name) {
+    if (!guiMainThread)
+        CV_Error( CV_StsNullPtr, "NULL guiReceiver (please create a window)" );
+
+    double result = 0;
+
+    QMetaObject::invokeMethod(guiMainThread,
+        "getWindowVisible",
+        autoBlockingConnection(),
+        Q_RETURN_ARG(double, result),
+        Q_ARG(QString, QString(name)));
+
+    return result;
+}
 
 void cvSetRatioWindow_QT(const char* name,double prop_value)
 {
@@ -903,6 +917,16 @@ double GuiReceiver::getPropWindow(QString name)
     return (double) w->getPropWindow();
 }
 
+double GuiReceiver::getWindowVisible(QString name)
+{
+    QPointer<CvWindow> w = icvFindWindowByName(name);
+
+    if (!w)
+        return 0;
+
+    return (double) w->isVisible();
+}
+
 
 void GuiReceiver::setPropWindow(QString name, double arg2)
 {