add String method (#552)

* all: add String() method for improved log output

highgui_string.go

@@ -0,0 +1,35 @@
+package gocv
+
+/*
+#include <stdlib.h>
+#include "highgui_gocv.h"
+*/
+import "C"
+
+func (c WindowFlag) String() string {
+	switch c {
+	case WindowNormal:
+		return "window-normal"
+	case WindowFullscreen:
+		return "window-fullscreen"
+	case WindowFreeRatio:
+		return "window-free-ratio"
+	}
+	return ""
+}
+
+func (c WindowPropertyFlag) String() string {
+	switch c {
+	case WindowPropertyFullscreen:
+		return "window-property-fullscreen"
+	case WindowPropertyAutosize:
+		return "window-property-autosize"
+	case WindowPropertyAspectRatio:
+		return "window-property-aspect-ratio"
+	case WindowPropertyOpenGL:
+		return "window-property-opengl"
+	case WindowPropertyVisible:
+		return "window-property-visible"
+	}
+	return ""
+}