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.cpp

@@ -73,6 +73,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "database/database-files.h"
 #include "database/database-dummy.h"
 #include "gameparams.h"
+#include "particles.h"
 
 class ClientNotFoundException : public BaseException
 {