ClipboardHistory: Add a clipboard history applet :^)

It will listen for clipboard content changes in the backgroud. Once you click
on its icon, it will pop up a window listing all recorded clipboard contents.
You can then double-click on an item to copy it again.

MenuApplets/ClipboardHistory/CMakeLists.txt

@@ -0,0 +1,8 @@
+set(SOURCES
+    ClipboardHistoryModel.cpp
+    IconWidget.cpp
+    main.cpp
+)
+
+serenity_bin(ClipboardHistory.MenuApplet)
+target_link_libraries(ClipboardHistory.MenuApplet LibGUI LibCore LibGfx)