Tidy up the QtQuick classes

- not part of the public API => private header naming convention
- unify class names (quick-prefix vs. win-prefix vs. quickwin-prefix)
  => drop the superfluous win-prefix from the class names, just like
     for the registered QML type names

Change-Id: I408cb22b38ae1db71e9bd7cdb8fa29c1aa1f504a
Reviewed-by: Ivan Vizir <define-true-false@yandex.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>