Taskbar: Add dropping of AppFiles to QuickLaunch

This change adds the capability to drop AppFiles to the quick launch
section of the Taskbar. All logic was moved to a new
QuickLaunchWidget.

Userland/Services/Taskbar/CMakeLists.txt

@@ -7,6 +7,7 @@ serenity_component(
 set(SOURCES
     main.cpp
     ClockWidget.cpp
+    QuickLaunchWidget.cpp
     ShutdownDialog.cpp
     TaskbarButton.cpp
     TaskbarWindow.cpp