Taskbar: Start working on a taskbar app.

I originally thought I would do this inside WindowServer, but let's try to
make it as a standalone app that communicates with WindowServer instead.
That will allow us to use LibGUI. :^)

Servers/WindowServer/WSWindowType.h

@@ -5,4 +5,5 @@ enum class WSWindowType {
     Normal,
     Menu,
     WindowSwitcher,
+    Taskbar,
 };