Kernel: Move all Graphics-related code into Devices/GPU directory

Like the HID, Audio and Storage subsystem, the Graphics subsystem (which
handles GPUs technically) exposes unix device files (typically in /dev).
To ensure consistency across the repository, move all related files to a
new directory under Kernel/Devices called "GPU".

Also remove the redundant "GPU" word from the VirtIO driver directory,
and the word "Graphics" from GraphicsManagement.{h,cpp} filenames.

Kernel/TTY/ConsoleManagement.cpp

@@ -8,7 +8,7 @@
 #include <Kernel/Boot/CommandLine.h>
 #include <Kernel/Debug.h>
 #include <Kernel/Devices/DeviceManagement.h>
-#include <Kernel/Graphics/GraphicsManagement.h>
+#include <Kernel/Devices/GPU/Management.h>
 #include <Kernel/Library/Panic.h>
 #include <Kernel/Sections.h>
 #include <Kernel/TTY/ConsoleManagement.h>