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/core/PasswordGenerator.cpp

@@ -25,15 +25,15 @@ const char* PasswordGenerator::DefaultExcludedChars = "";
 
 PasswordGenerator::PasswordGenerator()
     : m_length(0)
-    , m_classes(0)
-    , m_flags(0)
+    , m_classes(nullptr)
+    , m_flags(nullptr)
     , m_excluded(PasswordGenerator::DefaultExcludedChars)
 {
 }
 
 double PasswordGenerator::calculateEntropy(const QString& password)
 {
-    return ZxcvbnMatch(password.toLatin1(), 0, 0);
+    return ZxcvbnMatch(password.toLatin1(), nullptr, nullptr);
 }
 
 void PasswordGenerator::setLength(int length)

src/core/ScreenLockListenerPrivate.h

@@ -24,10 +24,10 @@ class ScreenLockListenerPrivate : public QObject
 {
     Q_OBJECT
 public:
-    static ScreenLockListenerPrivate* instance(QWidget* parent = 0);
+    static ScreenLockListenerPrivate* instance(QWidget* parent = nullptr);
 
 protected:
-    ScreenLockListenerPrivate(QWidget* parent = 0);
+    ScreenLockListenerPrivate(QWidget* parent = nullptr);
 
 signals:
     void screenLocked();

src/core/ScreenLockListenerWin.h

@@ -27,7 +27,7 @@ class ScreenLockListenerWin : public ScreenLockListenerPrivate, public QAbstract
 {
     Q_OBJECT
 public:
-    explicit ScreenLockListenerWin(QWidget* parent = 0);
+    explicit ScreenLockListenerWin(QWidget* parent = nullptr);
     ~ScreenLockListenerWin();
     virtual bool nativeEventFilter(const QByteArray& eventType, void* message, long*) override;