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/environment.h

@@ -35,8 +35,8 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include <atomic>
 #include <mutex>
 #include "irr_v3d.h"
-#include "network/networkprotocol.h" // for AccessDeniedCode
 #include "util/basic_macros.h"
+#include "line3d.h"
 
 class IGameDef;
 class Map;