Fix resetting cursor after dragging split separator

Reset the cursor to default, instead of setting to the value (arrow)
that happens to be the default. This fixes incorrect cursor in textctrls
under Windows.

src/edframe.cpp

@@ -148,7 +148,7 @@ class ThinSplitter : public wxSplitterWindow
             }
             else
             {
-                win->SetCursor(*wxSTANDARD_CURSOR);
+                win->SetCursor(wxNullCursor);
             }
         };
         win->Bind(wxEVT_MOTION, motionHandler);