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();

src/generic/grid.cpp

@@ -9659,6 +9659,11 @@ void wxGrid::Fit()
     AutoSize();
 }
 
+void wxGrid::SetFocus()
+{
+    m_gridWin->SetFocus();
+}
+
 #if WXWIN_COMPATIBILITY_2_8
 wxPen& wxGrid::GetDividerPen() const
 {