Fix "." filename in Save As dialog

If you hit Ctrl+s to save the image, then delete the filename and press
the cancel button, the next time you tried to Ctrl+s the same image the
filename would show as a period, ".". This patch fixes that - now it
will show the original filename.
Fixes #3174

rtgui/saveasdlg.h

@@ -57,7 +57,7 @@ public:
     bool            getToTailOfQueue   ();
     int             getSaveMethodNum   ();
 
-    void  setInitialFileName (Glib::ustring iname);
+    void  setInitialFileName (const Glib::ustring& iname);
     void  setImagePath (const Glib::ustring& imagePath);
 
     void okPressed ();