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.

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

Allow to set coutdown color from .countdown:selected for all themes
Fix the countdown for slider theme and draw transparent background for countdown

When loaded as in-process applet, we must not call `textdomain()` or we
will override mate-panel's text domain.  Instead, properly use
gi18n-lib.h header that provides the same API but uses the compile-time
`GETTEXT_PACKAGE` value for each call.

Actually drop the whole i18n setup logic that is provided already by
mate-panel's factory macros.

Fixes #219.