Neat little demo. It runs nicely and is quite playable. Very impressive what you've put together in a couple of days.
However, your documentation is lacking. And I'm also a big stickler for reducing code as much as is reasonable and making it extendable.
As your code stands, it's got a lot of repeated events (like a group for each box) which can be reduced to a fraction of the size - the 20 box event blocks can be reduced down to one generic block. Because of it's current form, it'll takes a bit of work to extend this demo. Want to more columns or rows? You've got to copy, paste & edit an existing block of events.
Please don't take this the wrong way.
I think your demo is great. And with a bit of code refactoring and commenting, it'd be fantastic as a template. If you do go ahead with this and want some help, contact me on the GDevelop forums (my handle is MrMen)