Fix linking of QtQuick.dll on Windows.

Fix linking of QtQuick.dll on Windows.

Put class QDeclarativeComponentAttached into a separate header.
When declared as Q_AUTOTEST_EXPORT in
declarative/qml/qdeclarativecomponent_p.h which is included by
quick/qquickloader.cpp, the Q_AUTOTEST_EXPORT is seen as
__declspec(dllexport) and linking fails.

Change-Id: I835197e3af6993cfd9325a432f33c636b9bfd3e6
Reviewed-by: Alan Alpert <alan.alpert@nokia.com>