Set focus to the main grid window in wxGrid::SetFocus()

After changing wxGrid to not inherit from wxPanel, this needs to be done
explicitly now.

include/wx/generic/grid.h

@@ -1948,6 +1948,7 @@ public:
 
     // override some base class functions
     virtual void Fit() wxOVERRIDE;
+    virtual void SetFocus() wxOVERRIDE;
 
     // implementation only
     void CancelMouseCapture();