Added different permission children for different gamemodes for /gamemode and /defaultgamemode

This is done as the same way as `/time`.

The gamemode-int-to-string thing code style was written with reference to [this part of PocketMine-MP code](https://github.com/PocketMine/PocketMine-MP/blob/master/src/pocketmine/event/HandlerList.php#L72).