Implement wxTreeCtrl::SetDoubleBuffered() in MSW wxTreeCtrl

Also don't erase background when the control is double-buffered because
it's not necessary and can cause flicker.

See https://github.com/wxWidgets/wxWidgets/pull/374

include/wx/msw/treectrl.h

@@ -202,6 +202,9 @@ public:
     // returns true if the platform should explicitly apply a theme border
     virtual bool CanApplyThemeBorder() const wxOVERRIDE { return false; }
 
+    virtual bool IsDoubleBuffered() const wxOVERRIDE;
+    virtual void SetDoubleBuffered(bool on) wxOVERRIDE;
+
 protected:
     // Implement "update locking" in a custom way for this control.
     virtual void DoFreeze() wxOVERRIDE;