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/clientmap.cpp

@@ -24,6 +24,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include <matrix4.h>
 #include "mapsector.h"
 #include "mapblock.h"
+#include "nodedef.h"
 #include "profiler.h"
 #include "settings.h"
 #include "camera.h"               // CameraModes