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.

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 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

Returning an error for a dbus method is counted as handling a method invocation. Return true in the handler to avoid a use-after-free.

remove exit / replace with gtk_main_quit

[ 7590.265881] mate-notificati[76519]: segfault at 55ac5111c67a ip 00007d1d4fc09d61 sp 00007ffd0bf06698 error 4 in libgobject-2.0.so.0.8000.0[7d1d4fbdc000+37000] likely on  CPU 0 (core 0, socket 0)
[ 7590.265910] Code: 01 00 00 00 4c 89 ce 48 89 e5 e8 da e6 ff ff 5d 85 c0 0f 95 c0 0f b6 c0 c3 f3 0f 1e fa 48 85 ff 74 47 48 8b 07 48 85 c0 74 3f <48> 8b 00 48 3d fc 03 00 00 77 2c 48 8d 15 cd 53 02 00 48 c1 e8 02

Fixes #218.