Add a feature which let us regulate our plant levels

@eieio_31310 I have the same issue with the player. We really need a system that allows to switch our plants levels (into level 1 at least).Using level high plants to finish level 1 level or using level 1 plants to defeat high level zombies in penny pursuit has no any fun and make this game boring.But most of players have upgraded their plants so the only way is to add such a system for dev.It is high time for them to do it next in the near future if they want this game to become better and attract more and bring back old players who have quitted pvz2.