Bitcoin-Qt: ensure all Windows scopes use win32 as identifier

- currently we sometimes use "windows" or "win32", but as per
  http://doc.qt.digia.com/qt/qmake-advanced-usage.html, windows seems to
  be not officially supported (only win32 is mentioned there)
Merge pull request #2109 from Diapolo/qt-pro-file-win32

Bitcoin-Qt: ensure all Windows scopes use win32 as identifier