Use `with-silent-modifications' instead of `org-unmodified' when it makes sense

Use `with-silent-modifications' instead of `org-unmodified' when it makes sense

* org-macs.el: Add a comment on when to use `org-unmodified'
and when to use `with-silent-modifications'.

* org-colview.el (org-columns-display-here)
(org-columns-remove-overlays, org-columns-quit)
(org-columns-edit-value, org-columns-compute-all)
(org-columns-compute, org-agenda-colview-compute):
* org-clock.el (org-clock-sum):
* org.el (org-refresh-category-properties)
(org-refresh-properties, org-entry-blocked-p)
(org-agenda-prepare-buffers): Use `with-silent-modifications'
instead of `org-unmodified'.

Thanks to Stefan Monnier for reminding me about `with-silent-modifications'!