Forces Windows (Vista and above) shutdown to wait qBittorrent to terminate cleanly

This pull request is related to issue #1535 

Starting from Vista, Microsoft has changed the shutdown management.
This patch tries to avoid the brutal kill of qBittorrent process before its clean termination.
Why Qt5-only restriction? If `installNativeEventFilter` only works in Qt5 maybe we can just ignore windows native events altogether and always defer shutdown from qbtsession destructor?