I initiated work on this but I do not think that I will bring the folder support to completion if nobody helps. If you want folder support to become reality, create a pull request for this branch. This proof-of-concept should make it easier to get started and help implementing the feature gradually instead of having to do everything at once.

• Subscriptions page crashes
• Sum up the counters of folders
• When scrolling quickly, folders sometimes get feed icons. Cancel glide loading when sharing ViewHolders
• Are layers only needed in the drawer, not the subscriptions screen? If so, move layer calculation to the drawer when making the flat list.
• Drawer items without a counter do not fill the whole width
• Add "tags" column to feed database table and store a list of folders (strings, separated by some nice ascii character like record separator)
• Add tags property to feed class
• Build folder list based on the tags property (can also just assign some tags before building as a proof-of-concept, no need for real tags from the database)
• Assign valid (and unique) item IDs to folders and items. This kind-of works but some animations are still broken.
• Long-pressing folders crashes
• Fix highlighting drawer items
• Sort folders to the end of the sidebar
• Store open folders across app launches
• Make layer depth dependent on dp instead of just assigning some pixel based padding. Maybe align images with the text from the parent folder?
• Show tags on top of the subscriptions screen (Allow applying tags to podcasts or putting them into groups/folders/categories #1711 (comment)) Postponed. Someone else should do that.
• Add nice UI for managing tags of a subscription (eg something like the new playback speed dialog with chips) It works but it's not nice. Someone else should do that.

Closes #1711