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/serveractiveobject.cpp

@@ -20,6 +20,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "serveractiveobject.h"
 #include <fstream>
 #include "inventory.h"
+#include "inventorymanager.h"
 #include "constants.h" // BS
 #include "log.h"
 
@@ -89,3 +90,8 @@ void ServerActiveObject::markForDeactivation()
 		m_pending_deactivation = true;
 	}
 }
+
+InventoryLocation ServerActiveObject::getInventoryLocation() const
+{
+	return InventoryLocation();
+}