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.

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.

Add persistence support with resident/transient hints, fix icon
precedence order (according to spec), and implement desktop-entry icons.

Also added new 'default-timeout' and 'enable-persistence' gsettings to
allow users to control timeout and persistence behavior.

Spec is on https://specifications.freedesktop.org/notification-spec/1.3/

Fixes https://github.com/mate-desktop/mate-notification-daemon/issues/132
Fixes https://github.com/mate-desktop/mate-notification-daemon/issues/137
Fixes https://github.com/mate-desktop/mate-notification-daemon/issues/138
Fixes https://github.com/mate-desktop/mate-notification-daemon/issues/149

Relying on D-Bus activation to launch org.freedesktop.Notifications can
result in selecting the wrong implementation if multiple daemons are
installed.

Fix this by launching m-n-d with the session rather than depending on
D-Bus activation.  We keep D-Bus activation for the moment for backward
compatibility and to re-start the daemon in otherwise non-problematic
situations if it crashes.

Fixes #133 and #174.

This allows to run as a regular session service that does not exit, but
also as a well-behaved D-Bus-activated service.  Make the default
behavior not to exit, but when activated through D-Bus.

Snap packages are unable to trigger the notification daemon in a fully
confined package, because the snapd apparmor rules state that the target
of the dbus call must specify that it is labelled `unconfined`. See
https://forum.snapcraft.io/t/snapd-doesnt-allow-notification-daemon-to-be-activatable/22912/1
for the details.

Signed-off-by: Dani Llewellyn <diddledani@ubuntu.com>