[WindowServer] SystemUIServer, LoginWindow: Deduplicate/Consolidate and Share Code

[RandomDSdevel]

     Currently, WindowServer's SystemUIServer's and LoginWindow's implementations duplicate some identical code that would, ideally, be consolidated and shared between them, likely in a shared framework private to WindowServer. This includes, but may not be limited to:

• Code for a 'Label' class. (Header: SystemUIServer, LoginWindow; implementation: SystemUIServer, LoginWindow.)
• Maybe some code for handling the log-in screen and desktop background window and window content output or (re-?)exposing an interface for doing that (mainly 'NSWindow' and 'AppDelegate' logic, though it's hard to say how much of the latter could be shared.) (Header: SystemUIServer, LoginWindow; implementation: SystemUIServer's 'desktop.h' and 'AppDelegate.m', LoginWindow's 'desktop.h', 'DesktopWindow.m', and 'AppDelegate.m'.)