Disable quota polling when default account does not exist or is offline
Query credentials when needed. Put the account offline if user aborts.

This is only implemented for HTTP auth. Shibboleth still does its own thing.
Prefix tooltips with app name
Fix initial state of quota info class.
Fix indentation
Introduce online/offline state, accessible via GUI
Remove credential-exposing debug output
docs: Add infos about installing the sql plugin and module

Fixes #1184
Distiguish "Signed out" from "Disconnected"
Cleanup: "Use QMutexLocker"