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/server/serverinventorymgr.h

@@ -20,8 +20,11 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #pragma once
 
 #include "inventorymanager.h"
+#include <cassert>
 #include <functional>
+#include <unordered_map>
 
+class IItemDefManager;
 class ServerEnvironment;
 
 class ServerInventoryManager : public InventoryManager