Allow files on desktop by adding DESKTOP folder

[inexorabletash]

Add a DESKTOP subdirectory in the A2D root that is presented as files on the desktop, in addition to volume icons.

This would be decidedly non-trivial

• Violates invariant that window id 0 implies "is volume"
• ... and also single root path for a given window
• Violates invariant that can't select both volumes and files simultaneously
• Path gets complicated when copied to RAMCard
• Icon locations are not persisted so you can't customize locations.

... but it'd be pretty cool.