Fix bug when update timestamps on filtered agenda entries

Fix bug when update timestamps on filtered agenda entries

* org-compat.el (org-in-invisibility-spec-p): Tiny code
cleanup.
(org-move-to-column): Only remove '(org-filtered) from
`buffer-invisibility-spec'.

* org-agenda.el (org-agenda-mode): Add buffer invisibility
specs.
(org-agenda-filter-hide-line, org-agenda-remove-filter): Set
the 'invisible text property to 'org-filtered.
(org-agenda-show-new-time): Add the default face to avoid the
foreground of the last character on the line to leak into the
timestamp notification.

Thanks to Thomas Morgan for reporting this.