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/unit_sao.cpp

@@ -21,6 +21,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "unit_sao.h"
 #include "scripting_server.h"
 #include "serverenvironment.h"
+#include "util/serialize.h"
 
 UnitSAO::UnitSAO(ServerEnvironment *env, v3f pos) : ServerActiveObject(env, pos)
 {