Change Q_WS_* to Q_OS_*

The Q_WS_{WIN,MAC} macros have been deprecated since Qt 4.3 or so,
and replaces by the corresponding Q_OS_* macros. Since Qt5 does not
support them anymore at all, let's globally change those to the "new"
version.

Also change Q_OS_WIN32 to just Q_OS_WIN; while the former is supposed to
be defined on all supported Windows platforms including Win64, it's still
a bit confusing.