Turn QtWin back to a namespace

The conditional QT_WIDGETS_LIB inline functions
won't work on MSVC when they are class members.
Symbols would have to be exported from the lib.
The enum duplication can be hopefully solved
later by adding Q_ENUMS() support for namespaces.

Change-Id: Ibf01e9c64db2be7e5f189f2088e879e9d8fabf54
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>