Replace all occurrences of the format string "%d:%02d" with
org-time-clocksum-format, which contains the same string by default.
This allows anyone to customize the appearance of clock summaries.
For example:
(setq org-time-clocksum-format "%02d:%02d")
There is an old XEmacs bug with indentation in the presence of
an extent with an invisibility property. We work around this now
by turning the invisibility property off, temporarily.