Improve backwards compatibility for Lua AI

This changes the following:
- Fixes the experimental AI, without changing any of its code except for that in the [engine] tag
- Returns a dummy self from the dummy Lua engine, so that external CAs are more easily switched to using [params]
- Changes the order in which parameters are passed to AI component code. The order is now:
        state/self,    params,    data

data/ai/lua/dummy_engine_lua.lua

@@ -7,4 +7,6 @@
 local ai_stdlib = wesnoth.require('ai/lua/stdlib.lua')
 ai_stdlib.init(ai)
 
--- No special state is returned by the default engine
+-- This is only returned for minor backwards compatibility
+local p, d = ...
+return {data = d}

data/core/macros/ai.cfg

@@ -548,8 +548,10 @@
     [engine]
         name="lua"
         code= <<
-            local ai = ...
-            return wesnoth.require("ai/lua/generic_rush_engine.lua").init(ai)
+        	local _,data = ...
+            local exp_ai = wesnoth.require("ai/lua/generic_rush_engine.lua").init(ai)
+            exp_ai.data = data
+            return exp_ai
         >>
     [/engine]
     [stage]

src/ai/composite/engine_lua.cpp

@@ -161,14 +161,14 @@ private:
 	bool use_parms_;
 
 	void generate_code(std::string& eval, std::string& exec) {
-		std::string preamble = "local params, data, state = ...\n";
+		std::string preamble = "local self, params, data = ...\n";
 		std::string load = "wesnoth.require(\"" + location_ + "\")";
 		if (use_parms_) {
 			eval = preamble + "return " + load + ":evaluation(ai, {" + eval_parms_ + "}, {data = data})";
 			exec = preamble + load + ":execution(ai, {" + exec_parms_ + "}, {data = data})";
 		} else {
-			eval = preamble + "return " + load + ".evaluation(params, data, state)";
-			exec = preamble + load + ".execution(params, data, state)";
+			eval = preamble + "return " + load + ".evaluation(self, params, data)";
+			exec = preamble + load + ".execution(self, params, data)";
 		}
 	}
 };

src/ai/lua/core.cpp

@@ -1050,7 +1050,7 @@ void lua_ai_context::update_state()
 	}
 	
 	// Store the state for use by components
-	lua_setfield(L, -2, "state"); // [-1: AI state]
+	lua_setfield(L, -2, "self"); // [-1: AI state]
 	
 	// And return with empty stack.
 	lua_pop(L, 1);
@@ -1127,9 +1127,10 @@ void lua_ai_action_handler::handle(const config &cfg, bool read_only, lua_object
 	lua_remove(L, -2); // [-1: AI action  -2: AI state]
 	
 	// Load the arguments
-	luaW_pushconfig(L, cfg); // [-1: parameters  -2: AI action  -3: AI state]
-	lua_getfield(L, -3, "data"); // [-1: data  -2: parameters  -3: action  -4: state]
-	lua_getfield(L, -4, "state");
+	int iState = lua_absindex(L, -2);
+	lua_getfield(L, iState, "self");
+	luaW_pushconfig(L, cfg);
+	lua_getfield(L, iState, "data");
 	
 	// Call the function
 	luaW_pcall(L, 3, l_obj ? 1 : 0, true);