Improve readability and type-safety

Use nullptr instead of 0 or NULL to initialize a null pointer. In some
cases, readability was enhanced by replacing 0 with more meaningful
values according to the type of the pointer being initialized.

src/gui/MessageWidget.h

@@ -28,7 +28,7 @@ class MessageWidget : public KMessageWidget
     Q_OBJECT
 
 public:
-    explicit MessageWidget(QWidget* parent = 0);
+    explicit MessageWidget(QWidget* parent = nullptr);
 
     int autoHideTimeout() const;