Clean up code (#3431)

* Empty destructors are replaced with default destructors
* A few loop variables made into const references to avoid copies
* Add missing `override` spec for some `Command::execute` methods

src/cli/Add.cpp

@@ -62,10 +62,6 @@ Add::Add()
     positionalArguments.append({QString("entry"), QObject::tr("Path of the entry to add."), QString("")});
 }
 
-Add::~Add()
-{
-}
-
 int Add::executeWithDatabase(QSharedPointer<Database> database, QSharedPointer<QCommandLineParser> parser)
 {
     TextStream inputTextStream(Utils::STDIN, QIODevice::ReadOnly);