Various bugfix (see #4008) :

1. RT could crash in some circumstances with single frame image
2. UNICODE UserComment now done when already existing in raw file
3. When editing UserComment in the ExifPanel, the Value filed is now
filled with the current value

rtexif/rtexif.h

@@ -290,9 +290,10 @@ public:
 
 
     // additional getter/setter for more comfortable use
-    std::string valueToString   ();
-    std::string nameToString    (int i = 0);
-    void        valueFromString (const std::string& value);
+    std::string valueToString         ();
+    std::string nameToString          (int i = 0);
+    void        valueFromString       (const std::string& value);
+    void        userCommentFromString (const Glib::ustring& text);
 
     // functions for writing
     int  calculateSize ();