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_inventory.h

@@ -106,7 +106,7 @@ private:
 public:
 	InvRef(const InventoryLocation &loc);
 
-	~InvRef();
+	~InvRef() = default;
 
 	// Creates an InvRef and leaves it on top of stack
 	// Not callable from Lua; all references are created on the C side.