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/browser/BrowserEntryConfig.h

@@ -35,7 +35,7 @@ class BrowserEntryConfig : public QObject
     Q_PROPERTY(QString Realm READ realm WRITE setRealm)
 
 public:
-    BrowserEntryConfig(QObject* object = 0);
+    BrowserEntryConfig(QObject* object = nullptr);
 
     bool load(const Entry* entry);
     void save(Entry* entry);