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/client/camera.h

@@ -22,6 +22,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "irrlichttypes_extrabloated.h"
 #include "inventory.h"
 #include "client/tile.h"
+#include "client/localplayer.h"
 #include <ICameraSceneNode.h>
 #include <ISceneNode.h>
 #include <plane3d.h>