As apps get large, they can start having a lot of pages. To make navigation in the editor easier, we’ve just added a way to define folders for pages. You can do this at the bottom of the Property Editor for pages and reusable elements.
Once you’ve created and used some folders, a dropdown will appear in the application page menu to let you filters pages based on the folder. Note this feature is edit-mode only, it has no impact on run mode.