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/walletmodel.cpp

@@ -701,7 +701,7 @@ bool WalletModel::bumpFee(uint256 hash)
     questionString.append("</td></tr></table>");
     SendConfirmationDialog confirmationDialog(tr("Confirm fee bump"), questionString);
     confirmationDialog.exec();
-    QMessageBox::StandardButton retval = (QMessageBox::StandardButton)confirmationDialog.result();
+    QMessageBox::StandardButton retval = static_cast<QMessageBox::StandardButton>(confirmationDialog.result());
 
     // cancel sign&broadcast if users doesn't want to bump the fee
     if (retval != QMessageBox::Yes) {