Refactor with C++17 guaranteed RVO

No need to declare undefined move constructors.
Only HouseEvent still needs the phony constructor due to NRVO.

- xml::StreamElement
- gui::test::SignalSpy

Issue #250

gui/tests/help.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2017 Olzhas Rakhimov
+ * Copyright (C) 2017-2018 Olzhas Rakhimov
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -87,8 +87,6 @@ class SignalSpy : public QObject,
         connect(sender, sig, this, &SignalSpy::accept, Qt::DirectConnection);
     }
 
-    SignalSpy(SignalSpy &&); ///< Only to enable RVO. Undefined.
-
 private:
     /// Stores the signal arguments.
     void accept(Ts... args) { this->emplace_back(args...); }

src/xml_stream.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2016-2017 Olzhas Rakhimov
+ * Copyright (C) 2016-2018 Olzhas Rakhimov
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -199,15 +199,6 @@ class StreamElement {
                 detail::FileStream* out)
       : StreamElement(name, 0, nullptr, indenter, out) {}
 
-  /// Move constructor is only declared
-  /// to make the compiler happy.
-  /// The code must rely on the RVO, NRVO, and copy elision
-  /// instead of this constructor.
-  ///
-  /// The constructor is not defined,
-  /// so the use of this constructor will produce a linker error.
-  StreamElement(StreamElement&&);
-
   /// Puts the closing tag.
   ///
   /// @pre No child element is alive.