Clean up header includes, add new forward declarations.

This cleans up many un-necessary header includes, either simply
providing nothing or forward declarations in their place.

No major compilation time change here, though this does reduce depencies
in some headers.

Also fix up old MacOS STL-include fixes that are no longer relevant.

Differential Revision: https://code.wildfiregames.com/D3128
This was SVN commit r24227.

source/scriptinterface/ScriptForward.h

@@ -0,0 +1,26 @@
+/* Copyright (C) 2020 Wildfire Games.
+ * This file is part of 0 A.D.
+ *
+ * 0 A.D. is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * 0 A.D. is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with 0 A.D.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef INCLUDED_SCRIPTFORWARD
+#define INCLUDED_SCRIPTFORWARD
+
+#include "js/TypeDecls.h"
+
+class ScriptInterface;
+class ScriptRequest;
+
+#endif // INCLUDED_SCRIPTFORWARD