aqt.util.tooltip(): use old help-hint.png size

Because this is actually an HTML-based overlay, it was mishandling a
higher resolution image and not scaling it down.

My initial thought for this was to specify width/height attributes on
it, but that made the image look really crummy (possibly because it is
not Qt proper).

This change puts back in the 32px version of the image, just for this
overlay.