proposed ui enhancements: multiple backends; bookmarks

[tdaff]

one should be able to use any installed backend at any time. Remote repositories should be remembered or be able to be bookmarked so that they appear in the list of available databases. Bookmarks could also be used to give shortened names to databases. For example:

usage: abcd [db-name] [selection] [options]
Hello. Databases you have access to:
>> asedb
   [my] my_first_db.db
   [short] long_name_for_database.db
   >> remote.ac.uk
      [collab] [email protected]:shared.db
      [collab2] [email protected]:shared_2.db
>> mongodb
   [db1] database_one
   database_no_short_name