Fixes for minor issues found by static analysis

Mostly style issues.

I used the following tools to find most of these:

- lgtm.com
- clang-tidy
- cpplint
- cppcheck

src/core/ScreenLockListenerWin.h

@@ -29,7 +29,7 @@ class ScreenLockListenerWin : public ScreenLockListenerPrivate, public QAbstract
 public:
     explicit ScreenLockListenerWin(QWidget* parent = nullptr);
     ~ScreenLockListenerWin();
-    virtual bool nativeEventFilter(const QByteArray& eventType, void* message, long*) override;
+    bool nativeEventFilter(const QByteArray& eventType, void* message, long*) override;
 
 private:
     void* m_powerNotificationHandle;