Migrate core source files to C++. Add yaml-cpp project as dependency (#2207)

By updating to this commit, Linux users have to regenerate Makefiles by re-running the configure script.

* Added yaml-cpp project as a 3rdparty library
* Migrated source files core to C++
* Updated configure scripts
* Make Linux installations compile *.cpp files
* Made server components' main source file C++
* Also made headers CPP-aware.

* Added basic C wrapper for yaml-cpp library
* YAML-node path is delimited by periods.
* Basic integer types and string are supported.
* Strings returned from this wrapper have to be freed with malloc.h::aFree
* Arrays (sequence) is supported with iterator wrapper.
* Remember to free every wrapper you create!

* Add yaml-cpp as dependency of common project
* Made the repo not ignore *.yml files

Thanks to @aleos89 and @Lemongrass3110.

3rdparty/yaml-cpp/include/yaml-cpp/null.h

@@ -0,0 +1,26 @@
+#ifndef NULL_H_62B23520_7C8E_11DE_8A39_0800200C9A66
+#define NULL_H_62B23520_7C8E_11DE_8A39_0800200C9A66
+
+#if defined(_MSC_VER) ||                                            \
+    (defined(__GNUC__) && (__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || \
+     (__GNUC__ >= 4))  // GCC supports "pragma once" correctly since 3.4
+#pragma once
+#endif
+
+#include "yaml-cpp/dll.h"
+#include <string>
+
+namespace YAML {
+class Node;
+
+struct YAML_CPP_API _Null {};
+inline bool operator==(const _Null&, const _Null&) { return true; }
+inline bool operator!=(const _Null&, const _Null&) { return false; }
+
+YAML_CPP_API bool IsNull(const Node& node);  // old API only
+YAML_CPP_API bool IsNullString(const std::string& str);
+
+extern YAML_CPP_API _Null Null;
+}
+
+#endif  // NULL_H_62B23520_7C8E_11DE_8A39_0800200C9A66