We started defining `_WIN32_WINNT` a little while ago to express that we
require Windows Vista or above for support. This enables us to access
some Windows Vista-only APIs. The fact that `WINVER` also needs to be
defined was missed, and it seems that defining one means the other no
longer gets defined automatically as it did before. Thus we're simplying
now also defining `WINVER` the same as `_WIN32_WINNT`.
This fixes High-DPI awareness and a few other things that were gated
behind WINVER checks.
This moves a lot of the preprocessor flags for what compiler/platform
we're using into a libqb-common.h header inside the libqb/include
folder. This gets included at the top of every libqb .cpp file, and is
intended to be fairly small, providing only necessary things like
_WIN32_WINNT (which needs to be defined before including <windows.h> or
friends).
