RFC5545 states that the DTEND and DUE properties must be specified with local ("floating") times if and only if DTSTART is local. This patch ensures that the incidence editor obeys that rule by setting the two timezone combo boxes to the same time zone if either of them changes to or from the "floating" time zone.
updateEndSpec()synchronize the combo boxes.
enableEndEdit()no longer change time zones when a todo's start or due time is enabled or disabled. Disabling one would set both to "floating"!
identical()ensures that all changes to time zones are detected.