Allow specifying a custom number of industries in map generation window

src/industry_cmd.cpp

@@ -2284,10 +2284,12 @@ static uint GetNumberOfIndustries()
 		25,   // low
 		55,   // normal
 		80,   // high
+		0,    // custom
 	};
 
 	assert(lengthof(numof_industry_table) == ID_END);
 	uint difficulty = (_game_mode != GM_EDITOR) ? _settings_game.difficulty.industry_density : (uint)ID_VERY_LOW;
+	if (difficulty == ID_CUSTOM) return std::min<uint>(IndustryPool::MAX_SIZE, _settings_game.game_creation.custom_industry_number);
 	return std::min<uint>(IndustryPool::MAX_SIZE, ScaleByMapSize(numof_industry_table[difficulty]));
 }