Android application

This project needs an Android application.
That would need making `TaxonomyLib` portable to Xamarin as a first step, and that needs:

- removing dependency on `SHA512Managed`
- replacement to `PathExt.GetRelativePath` function
- replacement to `Path.GetFullPath` function
- a way to access a SQLite database portably. Either a portable ADO.NET provider, or replacing the direct SQL queries.