OnFocus bug for hidden objects

[gdt050579]

OnFocus event is not triggered if a hidden object is asked to set focus. The main problem is that OnFocus is triggered only once during the Paint function and if an object is already hidden, it will not be trigered.
While the behavior is normal, it prevents to increase the size of an object that is already hidden.
The are a couple of solutions here:

1. modify the behavior of splitter to automatically increase its size if one of its childrens get focused
2. change the way OnFocus event is triggered (curently it is trigered only during Paint phase).