Reduce number of recursively included headers

This should improve compilation speed.

Things changed:
* Prefer forward-declarations in headers.
* Move header-includes out of headers if possible.
* Move some functions definitions out of headers.
* Put some member variables into unique_ptrs (see Client).

src/inventorymanager.h

@@ -19,10 +19,15 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 
 #pragma once
 
-#include "inventory.h"
+#include "irr_v3d.h"
 #include <iostream>
 #include <string>
+#include <vector>
+
 class ServerActiveObject;
+struct ItemStack;
+class Inventory;
+class IGameDef;
 
 struct InventoryLocation
 {