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/debuggermainwindow.h

@@ -70,7 +70,7 @@ public:
     Perspective() = default;
     // Takes ownership of \a centralWidget and all dock widgets in \a operations.
     Perspective(const QString &name, const QVector<Operation> &operations,
-                QWidget *centralWidget = 0);
+                QWidget *centralWidget = nullptr);
     ~Perspective();
 
     void addOperation(const Operation &operation);