WindowServer: Add a more generic mechanism for animations

This patch adds the WindowServer::Animation class, which represents
a simple animation driven by the compositor.

An animation has a length (in milliseconds) and two hooks:

- on_update: called whenever the animation should render something.
- on_stop: called when the animation is finished and/or stopped.

This patch also ports the window minimization animation to this new
mechanism. :^)

Userland/Services/WindowServer/CMakeLists.txt

@@ -10,6 +10,7 @@ compile_ipc(WindowManagerServer.ipc WindowManagerServerEndpoint.h)
 compile_ipc(WindowManagerClient.ipc WindowManagerClientEndpoint.h)
 
 set(SOURCES
+    Animation.cpp
     AppletManager.cpp
     Button.cpp
     ClientConnection.cpp