Add destroy notify callback to prevent use-after-free crashes when
notifications are closed before their reposition timeout fires.

Fixes #236

Add checks for monitor_id to prevent crash during monitor change events.
Instead it falls back to last available monitor and allows notification
processing to continue without a monitor.

Fixes: #200

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.

This adds visual feedback to the notification history by showing the
number of unread notifications in the buffer.

It also adds middle-click as a DND toggle.

Adds the basis for a notification management system using the D-Bus
context to display notification history in a popup.

Fixes https://github.com/mate-desktop/mate-notification-daemon/issues/74

Add D-Bus context module to connect future components of applet with
notification history.

Added a circular buffer for tracking notifications in dbus, and methods
for accessing those notifications. The idea is to implement a proper
notifications system.

Recent versions of libwnck introduced a WnckHandler object to be used as
the main entry point into the library.

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

Countdown was only visible for notifications that had action buttons.
This change adds a 'show-countdown' gsetting to display it instead on
all non-persisntent notifications.