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. :^)

LibGUI/GEventLoop.cpp

@@ -64,6 +64,7 @@ void GEventLoop::connect_to_server()
     request.greeting.client_pid = getpid();
     auto response = sync_request(request, WSAPI_ServerMessage::Type::Greeting);
     s_server_pid = response.greeting.server_pid;
+    GDesktop::the().did_receive_screen_rect(Badge<GEventLoop>(), response.greeting.screen_rect);
 }
 
 GEventLoop::GEventLoop()