NotificationServer: Add a system service for desktop notifications

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.

Base/etc/SystemServer.ini

@@ -20,6 +20,14 @@ Priority=low
 KeepAlive=1
 User=lookup
 
+[NotificationServer]
+Socket=/tmp/portal/notify
+SocketPermissions=660
+Lazy=1
+Priority=low
+KeepAlive=1
+User=notify
+
 [WindowServer]
 Socket=/tmp/portal/window
 SocketPermissions=660

Base/etc/group

@@ -5,5 +5,6 @@ phys:x:3:window
 audio:x:4:anon
 lookup:x:10:protocol,anon
 protocol:x:11:anon
-window:x:13:anon
+notify:x:12:anon
+window:x:13:anon,notify
 users:x:100:anon

Base/etc/passwd

@@ -1,6 +1,7 @@
 root:x:0:0:root:/:/bin/sh
 lookup:x:10:10:LookupServer,,,:/:/bin/false
 protocol:x:11:11:ProtocolServer,,,:/:/bin/false
+notify:x:12:12:NotificationServer,,,:/:/bin/false
 window:x:13:13:WindowServer,,,:/:/bin/false
 anon:x:100:100:Anonymous,,,:/home/anon:/bin/sh
 nona:x:200:200:Nona,,,:/home/nona:/bin/sh