Kernel+WindowServer: Move mouse input signal parsing to kernel driver.

It was silly for the WindowServer to have to know anything about the
format of PS/2 mouse packets.

This patch also enables use of the middle mouse button.

Kernel/MousePacket.h

@@ -0,0 +1,7 @@
+#pragma once
+
+struct MousePacket {
+    int dx { 0 };
+    int dy { 0 };
+    byte buttons;
+};