Make all notification log access go through dbus

This should fix issues where unread state wasn't getting noticed properly by the plugin or other entities, and makes log change notification granular and... well... just better.

Also this allows the daemon to "own" the log and be the only process that reads from and writes to it. This way, the plugin and config dialog can't ever break the DB through version incompatibilities, and failure modes will be better-handled.

Closes #90 (closed).