Add a new 'force-display' gsettings and checkbox that, when enabled,
shows all notifications regardless of urgency, fullscreen, or
screensaver state.  Do Not Disturb still takes precedence.

Fixes #199

This helps a bit with privacy. By disabling history, it wipes all
notifications in the D-Bus buffer and prevents further storage. The
history popup is also disabled.

Notification storage resumes when history is enabled.

Note that this just prevents the mate-notifications-daemon from storing
notifications, but it doesn't prevent other processes from capturing
them elsewhere.

Add new UI elements to configure default timeout, allow persistent
notifications, and to display countdown on all notification messages.