iCalendar import: update the object too when "last-modified" was not changed.

According to http://tools.ietf.org/search/rfc5545#section-3.8.7.3, the
"last-modified" property contains the date/time of the last change in the calendar
store (server side).  Thus, the client is not obliged to update it.
rename for more meaningful names
Merge branch 'jone-ical-import-last-modified' into plone5
clearer changelog note, reformat long lines