Use static_cast instead of C-style casts for non-fundamental types

A C-style cast is equivalent to try casting in the following order:

1. const_cast(...)
2. static_cast(...)
3. const_cast(static_cast(...))
4. reinterpret_cast(...)
5. const_cast(reinterpret_cast(...))

By using static_cast<T>(...) explicitly we avoid the possibility
of an unintentional and dangerous reinterpret_cast. Furthermore
static_cast<T>(...) allows for easier grepping of casts.

src/qt/coincontroltreewidget.cpp

@@ -24,7 +24,7 @@ void CoinControlTreeWidget::keyPressEvent(QKeyEvent *event)
     else if (event->key() == Qt::Key_Escape) // press esc -> close dialog
     {
         event->ignore();
-        CoinControlDialog *coinControlDialog = (CoinControlDialog*)this->parentWidget();
+        CoinControlDialog *coinControlDialog = static_cast<CoinControlDialog*>(this->parentWidget());
         coinControlDialog->done(QDialog::Accepted);
     }
     else