Made QWindowsXPStyle internal

Made QWindowsXPStyle internal

We will take this opportynity to remove various QStyle specializations
from the public API in Qt5. This gives us much more freedom, for
example changing the inheritance hierarchy and even merging style
implementations (XP & Vista) later on, without worrying about BC.

=> Use QStyleFactory and/or QProxyStyle instead of creating an instance
or inheriting QWindowsXPStyle directly.

Change-Id: Ib4fec35c2fcb3b5a193537a1de5b6e26fbbe0a06
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>