Portable serialization

[skgrush]

Resolves #30.

Primary changes

• upgraded ScanFile aka .cavernseerscan to v2
  • changed ARAnchor classes we were serializing to custom portable representations which can be used on devices that don't support ARKit.
  • meshAnchors from ARMeshAnchor to CSMeshSlice
  • startSnapshot and endSnapshot from SnapshotAnchor to CSMeshSnapshot
  • opening v1 files automatically converts them to v2

Secondary changes

• added Metadata section to Advanced menu showing id, name, timestamp, encodingVersion, url, and fileSize of the ScanFile
• reorganize ScanFile decoding to be more flexible.
• added an option in Advanced menu for "Copy and Rename" which can also upgrade existing v1 files non-destructively
• allow permanently deleting all files stored; useful for debugging but maybe not worth it?
• fix setVisible, now waits for refresh and switches the view to the passed scan
• allow copying/sharing start and end snapshots via context menu
• tests for encoding and decoding

TODO before merge

• remove ScanFile.currentEncodingVersion experimental title (keep v2 documentation)