Add (optional) connection logging

[spodskubka]

Analyzing connection failures is hard to do without some kind of logging built into the app.

• Logging should be optional (globally or per connection)
• Logging should not contain sensitive data (passwords, private keys, transferred data), or have configurable detail
• Logging should be done in a temporary file in the app storage
• On the user's request the log should be exported to a plain text file