Kernel/HID: Expose character map index in the KeyEvent structure

This will be used later on by WindowServer so it will not use the
scancode, which will represent the actual character index in the
keyboard mapping when using scan code set 2.

Kernel/API/KeyCode.h

@@ -142,6 +142,7 @@ enum KeyModifier {
 
 struct KeyEvent {
     KeyCode key { Key_Invalid };
+    u8 map_entry_index { 0 };
     u32 scancode { 0 };
     u32 code_point { 0 };
     u8 flags { 0 };