StButton: fix hover and grab tracking

StButton: fix hover and grab tracking

Use StWidget:track-hover rather than doing it ourselves. Don't assume
that hover is always TRUE after an enter_event or FALSE after a
leave_event, since we have a pointer grab and will be getting other
actors' events.

Don't ungrab the pointer when it leaves the button, since that
destroys the whole point of getting a grab in the first place.

Only consider the button to have been clicked when it has both grab
(meaning the mouse was pressed over the button) and hover (meaning the
mouse was released over the button).

Also remove the virtual pressed/released methods, which weren't being
used anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=633853