[libcalamaresui] Also modern-style initialize center and disable flags

- All constructors explicitly initialize these, so there's no
  real change here.

src/libcalamaresui/widgets/waitingspinnerwidget.h

@@ -150,8 +150,8 @@ private:
     QColor  _textColor = Qt::black;
 
     // Environment settings
-    bool    _centerOnParent;
-    bool    _disableParentWhenSpinning;
+    bool    _centerOnParent = true;
+    bool    _disableParentWhenSpinning = true;
 
     // Internal bits
     QTimer *_timer = nullptr;