Logging improvements
Improves both the engine's and client's logging handling.
- Replaces the engine's custom logging flags with logging sub-domains
- Gets more engine objects implementing the
Logging.Source
interface - Create log records for all logged messages, so post-hoc filtering works
- Removes obsolete client command line flags, split up args for enabling IMAP/SMTP protocol logging and SQL queries and results
- Adds a sidebar to the Inspector's log pane, allow filtering by account and logging domain
- Adds a mark and clear button to the Inspector's log pane
- Ensures Inspector marks are always displayed in the logging pane
- Suppresses GdkPixbuf and GLib-Net logging by default