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/debugger/stackwindow.cpp

@@ -133,7 +133,7 @@ void StackWindow::copyContentsToClipboard()
         str += '\n';
     }
     QClipboard *clipboard = QApplication::clipboard();
-    #ifdef Q_OS_LINUX
+    #ifdef Q_WS_X11
     clipboard->setText(str, QClipboard::Selection);
     #endif
     clipboard->setText(str, QClipboard::Clipboard);