feature: updating apps without removing app folders

[quanglam2807]

Would it be possible for WebCatalog to not be required to empty the entire folder? I do want to keep a couple of non-related files inside the apps folders. One example is the folder icon, another example is a "VisualElements" file and folder, which stylizes the icons in my start menu (via the app called "Tile Iconifier". These files do not cause any harm being in the folder with the app, and would be nice if these files would persist across app updates, instead of being deleted when the app is updated.

Originally posted by @profucius in #1381 (comment)