input: add key name for U+3000 IDEOGRAPHIC SPACE

Deserves its own name, because just like SPACE it's a printable
character, but invisible.

Fixes #2349... I think.

input/keycodes.c

@@ -36,6 +36,7 @@ struct key_name {
 static const struct key_name key_names[] = {
   { ' ', "SPACE" },
   { '#', "SHARP" },
+  { 0x3000, "HALF_SPACE" },
   { MP_KEY_ENTER, "ENTER" },
   { MP_KEY_TAB, "TAB" },
   { MP_KEY_BACKSPACE, "BS" },