[Windows] Also check for QApplication::style when animating buttons.

The fallback style for a custom style sheet is QApplication::style(),
which defaults to QWindowsVistaStyle on most (if not all)  windows
systems.

Task-number: QTBUG-39400
Change-Id: Ib05571033730ed26a39f89d8e9fe948c4b280e64
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>