Code modernization: subfolders (#6283)

* Code modernization: subfolders

Modernize various code on subfolders client, network, script, threading, unittests, util

* empty function
* default constructor/destructor
* for range-based loops
* use emplace_back instead of push_back
* C++ STL header style
* Make connection.cpp readable in a pointed place + typo

src/script/lua_api/l_item.h

@@ -121,7 +121,7 @@ private:
 
 public:
 	LuaItemStack(const ItemStack &item);
-	~LuaItemStack();
+	~LuaItemStack() = default;
 
 	const ItemStack& getItem() const;
 	ItemStack& getItem();