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/projectexplorer/projecttreewidget.cpp

@@ -90,7 +90,7 @@ protected:
             QTreeView::focusOutEvent(event);
     }
 
-#ifdef Q_OS_MAC
+#ifdef Q_WS_MAC
     void keyPressEvent(QKeyEvent *event)
     {
         if ((event->key() == Qt::Key_Return