New star shop and proper level setup

Back to it after the holidays. This time, I did a lot of setup and organisation work, so not much to show. 

1) I made a proper level list (with some empty space) that will let me add new levels properly without having to rearrange everything each time.
2) Levels now remember and display the stars you've collected, and these stars are saved and added to your star count.

3) Added a new spot on the world map, Vi's shop, where you can talk to Vi the new npc to access the shop. From there, you can trade your stars to unlock bonus levels. 

For now there's only one level to unlock and another one on display for show. 

I'm not too sure about what I'll work on in the future, probably finishing and polishing the first worlds, or I might get to work on the UI. There are some marketting tasks I must do as well.