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/transactionview.cpp

@@ -309,7 +309,7 @@ void TransactionView::chooseWatchonly(int idx)
     if(!transactionProxyModel)
         return;
     transactionProxyModel->setWatchOnlyFilter(
-        (TransactionFilterProxy::WatchOnlyFilter)watchOnlyWidget->itemData(idx).toInt());
+        static_cast<TransactionFilterProxy::WatchOnlyFilter>(watchOnlyWidget->itemData(idx).toInt()));
 }
 
 void TransactionView::changedPrefix(const QString &prefix)