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/unittest/mock_inventorymanager.h

@@ -18,6 +18,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 */
 
 #include <gamedef.h>
+#include <inventory.h>
 #include <inventorymanager.h>
 
 class MockInventoryManager : public InventoryManager