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/debuggeractions.cpp

@@ -106,7 +106,7 @@ DebuggerSettings::DebuggerSettings()
     const QString debugModeGroup = QLatin1String(debugModeSettingsGroupC);
     const QString cdbSettingsGroup = QLatin1String(cdbSettingsGroupC);
 
-    SavedAction *item = 0;
+    SavedAction *item = nullptr;
 
     item = new SavedAction(this);
     insertItem(SettingsDialog, item);