Mainmenu: Move core.on_before_close to s_mainmenu like other callbacks, and doc

src/script/cpp_api/s_mainmenu.cpp

@@ -34,7 +34,7 @@ void ScriptApiMainMenu::handleMainMenuEvent(const std::string &text)
 	lua_getfield(L, -1, "event_handler");
 	lua_remove(L, -2); // Remove core
 	if (lua_isnil(L, -1)) {
-		lua_pop(L, 1); // Pop event_handler
+		lua_pop(L, 2); // Pop event_handler, error handler
 		return;
 	}
 	luaL_checktype(L, -1, LUA_TFUNCTION);
@@ -56,7 +56,7 @@ void ScriptApiMainMenu::handleMainMenuButtons(const StringMap &fields)
 	lua_getfield(L, -1, "button_handler");
 	lua_remove(L, -2); // Remove core
 	if (lua_isnil(L, -1)) {
-		lua_pop(L, 1); // Pop button handler
+		lua_pop(L, 2); // Pop button handler, error handler
 		return;
 	}
 	luaL_checktype(L, -1, LUA_TFUNCTION);
@@ -76,3 +76,25 @@ void ScriptApiMainMenu::handleMainMenuButtons(const StringMap &fields)
 	PCALL_RES(lua_pcall(L, 1, 0, error_handler));
 	lua_pop(L, 1); // Pop error handler
 }
+
+void ScriptApiMainMenu::beforeClose()
+{
+	SCRIPTAPI_PRECHECKHEADER
+
+	int error_handler = PUSH_ERROR_HANDLER(L);
+
+	// Get handler function
+	lua_getglobal(L, "core");
+	lua_getfield(L, -1, "on_before_close");
+	lua_remove(L, -2); // Remove core
+	if (lua_isnil(L, -1)) {
+		lua_pop(L, 2); // Pop callback, error handler
+		return;
+	}
+	luaL_checktype(L, -1, LUA_TFUNCTION);
+
+	// Call it
+	PCALL_RES(lua_pcall(L, 0, 0, error_handler));
+
+	lua_pop(L, 1); // Pop error handler
+}

src/script/cpp_api/s_mainmenu.h

@@ -27,4 +27,9 @@ public:
 	 * @param fields data in field format
 	 */
 	void handleMainMenuButtons(const StringMap &fields);
+
+	/**
+	 * Called before the menu is closed, either to exit or to join a game
+	 */
+	void beforeClose();
 };