Unlocking a custom 'Next' button after user visits each layer

What Judy said. :)

Nice looking interaction. 

I'd simplify your triggers:

• Get rid of the change states to hide object triggers. Instead set the initial state of those objects to hidden. 
• You don't need a trigger to change a state to visited when a user clicks an object since that will automatically happen (that's on the layers). You're using those visited states for conditions. But you don't need to.
• Most of the objects are images and have visited states. There are a few grouped objects that can't have visited states since they're in a group.
• I'd select on of the objects in the group and add a visited state so you can change it to visited like the non-grouped objects.
• Then use a trigger to change the state of the continue button when the states of all the objects are visited.