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/sendcoinsdialog.cpp

@@ -352,7 +352,7 @@ void SendCoinsDialog::on_sendButton_clicked()
     SendConfirmationDialog confirmationDialog(tr("Confirm send coins"),
         questionString.arg(formatted.join("<br />")), SEND_CONFIRM_DELAY, this);
     confirmationDialog.exec();
-    QMessageBox::StandardButton retval = (QMessageBox::StandardButton)confirmationDialog.result();
+    QMessageBox::StandardButton retval = static_cast<QMessageBox::StandardButton>(confirmationDialog.result());
 
     if(retval != QMessageBox::Yes)
     {