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/script/cpp_api/s_modchannels.h

@@ -20,7 +20,8 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #pragma once
 
 #include "cpp_api/s_base.h"
-#include "modchannels.h"
+
+enum ModChannelSignal : u8;
 
 class ScriptApiModChannels : virtual public ScriptApiBase
 {