Initial Notificiation Applet - introduce the dbus interface

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Commit message changed for mainline inclusion
Signed-off-by: Ikey Doherty <ikey.doherty@gmail.com>
applets/notifications: Minor UI padding cleanup and code fixes
applets/notification: Show a notification widget on incoming message
applets/notifications: Major refactor, more robust.

We now handle notifications in a single idle management function, which
only performs when notifications are present. As soon as all notifications
have disappeared we disconnect our timeout.
Notifications match the freedesktop spec by ensuring a unique ID for
each sender message, allowing clients to replace one of their existing
notifications with a new one, i.e. for music players that merely wish
to update a potentially visible playlist notification.
applets/notifications: Remove deprecated field
data: Drop border styling - looks silly
applets/notifications: Enable dismissing of notifications
applets/notifications: Implement urgency support
applets/notifications: Set a fallback icon and improve spacing
applets/notifications: Fix uint32 hiccup.
applets/clock: Respect GNOME clock settings