ifdef Q_?S_* cleanup

- clear up some Q_OS_LINUX vs. Q_OS_UNIX
- clear up Q_WS_* vs. Q_OS_* (relatively theoretical exercise)
- use proper #if-#else cascades instead of free-standing #ifs

src/plugins/coreplugin/fileiconprovider.cpp

@@ -70,7 +70,7 @@ QIcon FileIconProvider::icon(const QFileInfo &fileInfo)
 
         // Disabled since for now we'll make sure that all icons fit with our
         // own custom icons by returning an empty one if we don't know it.
-#ifdef Q_OS_WIN
+#ifdef Q_WS_WIN
         // This is incorrect if the OS does not always return the same icon for the
         // same suffix (Mac OS X), but should speed up the retrieval a lot ...
         icon = m_systemIconProvider.icon(fileInfo);
@@ -123,7 +123,7 @@ void FileIconProvider::registerIconOverlayForSuffix(const QIcon &icon, const QSt
 QIcon FileIconProvider::iconForSuffix(const QString &suffix) const
 {
     QIcon icon;
-#ifndef Q_OS_WIN // On windows we use the file system icons
+#ifndef Q_WS_WIN // On windows we use the file system icons
     if (suffix.isEmpty())
         return icon;