VR keyboard: small Entf hint on the Delete key without AltGr

The key that becomes Delete on AltGr shows a small hint (the arrow hints'
7 px) on the baseline of Steam's AltGr hints, while the layout has such a
free key.

README.md

@@ -461,8 +461,8 @@ reboot or Steam restart needed.
   Shift + arrows select text.
 - AltGr + the key left of Backspace (`´` on German, `=` on US): Delete,
   labelled like Steam's Delete key in its language (`Entf`; `Del` if that is
-  longer); repeats while held. Layouts with an AltGr character on that key
-  get none.
+  longer), hinted on the key without AltGr; repeats while held. Layouts with
+  an AltGr character on that key get none.
 - Ctrl/Alt chords and Esc are sent with `xdotool key` on `:0` (focus follows
   the VR-selected window); a toggled Ctrl/Alt is held down while the
   keyboard is open (e.g. Ctrl+scroll).