Don't stack notifications on top of each other, instead put them below
one another on the y axis.
This will obviously break if the screen fills with notifications, but
that's a FIXME for now. :^)
This patch adds NotificationServer, which runs as the "notify" user
and provides an IPC API for desktop notifications.
LibGUI gains the GUI::Notification class for showing notifications.
NotificationServer is spawned on demand and will unspawn after
dimissing all visible notifications. :^)
Finally, this also comes with a small /bin/notify utility.
