Extend Android app to support browsing directories

The deliverable is as above with the following additions. If a CHK or SDMF capability represents a directory object then the directory contents are interpreted and a structured representation of them is displayed in the GUI. If a child of the directory is a directory then the GUI UX can recurse into that child.

1. Finalize GUI design
2. Integrate Tahoe-LAFS Haskell client libraries with existing Kotlin GUI or new Obelisk-based GUI