WindowServer: Introduce the ScreenBackend concept

The ScreenBackend is a thin wrapper around the actual screen hardware
connection. It contains all the variables specific to that hardware and
abstracts away operations that deal with controlling the hardware. The
standard ScreenBackend implementor is HardwareScreenBackend, which
contains all the existing frame buffer & ioctl handling code of Screen.
I took this opportunity to introduce ErrorOr wherever sensible.

Userland/Services/WindowServer/CMakeLists.txt

@@ -25,6 +25,7 @@ set(SOURCES
     MultiScaleBitmaps.cpp
     Overlays.cpp
     Screen.cpp
+    HardwareScreenBackend.cpp
     ScreenLayout.cpp
     Window.cpp
     WindowFrame.cpp