Taskbar: Integrate clock widget into taskbar window

Instead of running the clock widget as a separate menu applet,
let's just draw a clock here in the taskbar window.

Userland/Services/Taskbar/CMakeLists.txt

@@ -1,5 +1,6 @@
 set(SOURCES
     main.cpp
+    ClockWidget.cpp
     TaskbarButton.cpp
     TaskbarWindow.cpp
     WindowList.cpp