Debugger: Modernize

Mostly nullptr instead of 0, but also a few bits of collateral damage.

Change-Id: I921991272aca921dcdecf302dfff3716e79dfc24
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/debugger/imageviewer.cpp

@@ -49,7 +49,7 @@ class ImageWidget : public QWidget
 {
     Q_OBJECT
 public:
-    explicit ImageWidget(QWidget *parent = 0) : QWidget(parent) {}
+    ImageWidget() {}
 
     void setImage(const QImage &image);
     const QImage &image() const { return  m_image; }