WindowServer: Create the VirtualScreenBackend

This screen backend is just memory-backed and doesn't connect to any
screen hardware. That way, we can boot Serenity without video hardware
but in full graphical mode :^)

To create a virtual screen, put something like this in your
WindowServer.ini. There's no way yet to do this through Display
Settings, though an existing virtual screen's settings can be changed
there.
```ini
[Screen0]
Mode=Virtual
Left=1024
Top=0
Width=1920
Height=1080
ScaleFactor=1
```

Userland/Services/WindowServer/CMakeLists.txt

@@ -26,6 +26,7 @@ set(SOURCES
     Overlays.cpp
     Screen.cpp
     HardwareScreenBackend.cpp
+    VirtualScreenBackend.cpp
     ScreenLayout.cpp
     Window.cpp
     WindowFrame.cpp