ThickClient: refactor BaseClientWindow

* No need for a ctor of an abstract class to be public.
* Remove unneeded 'using' clauses.
* The ConfigureWindow() method was protected virtual but:
- Nobody was overriding it.
- Nobody was calling it.
So now it can be private and non-virtual, and we can encapsulate
some logic in it related to window configuration (previously it
was empty).