Do not require restart when chaning Background pause option

Background pause does require a game restart to take effect.
Adding function to update it on runtime since only place where it is
used is in main.cpp.

Differential revision: D4181
Fixes: #6236
Tested by: @Langbart
This was SVN commit r25866.

binaries/data/mods/public/gui/options/options.json

@@ -19,7 +19,8 @@
 				"type": "boolean",
 				"label": "Background pause",
 				"tooltip": "Pause single-player games when window loses focus.",
-				"config": "pauseonfocusloss"
+				"config": "pauseonfocusloss",
+				"function": "PauseOnFocusLoss"
 			},
 			{
 				"type": "boolean",

source/ps/scripting/JSInterface_ConfigDB.cpp

@@ -188,6 +188,11 @@ bool SetFile(const std::wstring& cfgNsString, const Path& path)
 	return true;
 }
 
+void PauseOnFocusLoss(bool pause)
+{
+	g_PauseOnFocusLoss = pause;
+}
+
 void RegisterScriptFunctions(const ScriptRequest& rq)
 {
 	ScriptFunction::Register<&HasChanges>(rq, "ConfigDB_HasChanges");
@@ -201,5 +206,6 @@ void RegisterScriptFunctions(const ScriptRequest& rq)
 	ScriptFunction::Register<&CreateAndWriteValueToFile>(rq, "ConfigDB_CreateAndWriteValueToFile");
 	ScriptFunction::Register<&SetFile>(rq, "ConfigDB_SetFile");
 	ScriptFunction::Register<&Reload>(rq, "ConfigDB_Reload");
+	ScriptFunction::Register<&PauseOnFocusLoss>(rq, "PauseOnFocusLoss");
 }
 }

source/ps/scripting/JSInterface_ConfigDB.h

@@ -1,4 +1,4 @@
-/* Copyright (C) 2019 Wildfire Games.
+/* Copyright (C) 2021 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -18,6 +18,7 @@
 #ifndef INCLUDED_JSI_CONFIGDB
 #define INCLUDED_JSI_CONFIGDB
 
+extern bool g_PauseOnFocusLoss;
 class ScriptRequest;
 
 namespace JSI_ConfigDB