Kernel/Memory: Introduce the SharedFramebufferVMObject class

This new type of VMObject will be used to coordinate switching safely
from graphical mode to text mode and vice-versa, by supplying a way to
remap all Regions that were created with this object, so mappings can be
changed according to the given state of system mode. This makes it quite
easy to give applications like WindowServer the feeling of having full
access to the framebuffer device from a DisplayConnector, but still keep
the Kernel in control to be able to safely switch to text console.

Kernel/Forward.h

@@ -25,6 +25,7 @@ class DiskCache;
 class DoubleBuffer;
 class File;
 class OpenFileDescription;
+class DisplayConnector;
 class FileSystem;
 class FutexQueue;
 class IPv4Socket;