AI: move default algorithm definition from C++ to WML

Among other things this now allows defining cores that have a different default AI.

data/_main.cfg

@@ -38,6 +38,7 @@
 {campaigns/}
 
 [ais]
+    default_ai_algorithm=ai_default_rca
     [default_config]
         {ai/utils/default_config.cfg}
     [/default_config]

src/ai/configuration.cpp

@@ -49,6 +49,10 @@ void configuration::init(const config &game_config)
 		ERR_AI_CONFIGURATION << "Missing AI [default_config]. Therefore, default_config_ set to empty." << std::endl;
 		default_config_.clear();
 	}
+	default_ai_algorithm_ = ais["default_ai_algorithm"].str();
+	if (default_ai_algorithm_.empty()) {
+		ERR_AI_CONFIGURATION << "Missing default_ai_algorithm. This will result in no AI being loaded by default." << std::endl;
+	}
 
 
 	for (const config &ai_configuration : ais.child_range("ai")) {
@@ -166,6 +170,7 @@ configuration::description_map configuration::ai_configurations_ = configuration
 configuration::description_map configuration::era_ai_configurations_ = configuration::description_map();
 configuration::description_map configuration::mod_ai_configurations_ = configuration::description_map();
 config configuration::default_config_ = config();
+std::string configuration::default_ai_algorithm_;
 
 bool configuration::get_side_config_from_file(const std::string& file, config& cfg ){
 	try {
@@ -359,7 +364,7 @@ void configuration::expand_simplified_aspects(side_number side, config &cfg) {
 		}
 	}
 	if (algorithm.empty() && !parsed_config.has_child("stage")) {
-		base_config = get_ai_config_for("ai_default_rca");
+		base_config = get_ai_config_for(default_ai_algorithm_);
 	}
 	for (const config::any_child &child : parsed_config.all_children_range()) {
 		base_config.add_child(child.key, child.cfg);

src/ai/configuration.hpp

@@ -120,6 +120,7 @@ class configuration {
 	static description_map era_ai_configurations_;
 	static description_map mod_ai_configurations_;
 	static config default_config_;
+	static std::string default_ai_algorithm_;
 
 };