increase clickable area of vbox-next and vbox-prev
remove duplicate navigation code

The navigation for the arrow keys and the nav buttons consisted on
duplicate code, which is not a good idea from a maintenance and
debugging perspective.

This commit fixes the duplicate code by adding a `gallnav` object,
containing the common navigation code, which is reused on the
different sections where it's needed.
unbind escape handler after closeVbox() is called

An escape key handler is bound to the keydown event of the body,
which in turn calls closeVbox() to dismiss the lightbox.

This commit ensures the escape handler is unbound when closeVbox()
is called, because it's no longer needed.
allow using a custom attribute for the lightbox title

Even though [title] is apropriate for most use cases, it would be good
to have some flexibility and allow the use of another attribute for
the lightbox title (e.g. [data-title]). This is for cases in which
the tooltip does not make sense.

This commit introduces the `titleattr` option (which defaults to
'title'), to allow such flexibility.