org-agenda.el: Fix bug when using `org-agenda-do-date-later' in filtered agendas

org-agenda.el: Fix bug when using `org-agenda-do-date-later' in filtered agendas

* org-agenda.el (org-agenda-filter-hide-line): Hide from the
beginning of the line to the beginning of the next line.
(org-agenda-show-new-time): Use `move-beginning-of-line' and
`move-end-of-line'.
(org-agenda-drag-line-forward): Adapt to the new definition of
hidden filtered lines.

Thanks to Igor Sosa Mayor for reporting this bug and to Nick Dokos
and Matt Lundin for providing further information.