Windows: Start using High DPI API available from Windows 8.1 onwards.

Dynamically load shcore.dll, use SetProcessDpiAwareness() instead
of SetProcessDPIAware() when available. Add command line parameter
to control level.

Task-number: QTBUG-37347
Change-Id: I1259b0943b41e50066e7e3870ed3136afd8f18fe
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>