Add onButtonClick to take over the hint button

The built-in button always dismissed; repurposing it meant hiding it and
rebuilding one in onPopoverRender. onButtonClick (per hint or instance
wide, hint wins) now runs instead of dismissing, mirroring how a tour's
onNextClick takes over the default advance; call dismiss() from the hook
to keep the old behavior. Custom and extra buttons remain the
onPopoverRender pattern, same as tours.