org-agenda.el: Fix bug when displaying the changed date

org-agenda.el: Fix bug when displaying the changed date

* org-agenda.el (org-agenda-mode): Disable `indent-tabs-mode'.
(org-agenda-dim-blocked-tasks): Make overlays intangible.
(org-agenda-show-new-time): Fix bug when deleting a timestamp
right after changing it.
(org-agenda-check-for-timestamp-as-reason-to-ignore-todo-item)
(org-agenda-get-sexps, org-agenda-filter-by-regexp)
(org-agenda-filter-top-headline-apply): Fix indentation.

Thanks to Fletcher Charest and Nick Dokos for reporting issues
in this area.

There is one bug left: in a filtered agenda, changing a timestamp
will not always correctly display the warning.  This happens when
the headline at point is followed by hidden headlines, because
`org-agenda-show-new-time' tries to set a text property at a
point where there is an overlay that hides then text.  Because
the overlay is now intangible, the warning text will be displayed
too far on the right, and may be outside of the window.