when user changes event timezone, start/end same time in new timezone

as opposed to silently and invisibly expressing the absolute point
in time in new timezone.

E.g. if the event was:

start: 14 February 19:00 (invisibly: UTC+2)
end:   14 February 23:00 (invisibly: UTC+2)
tz:    UTC+2

and the users changes the tz to UTC+3, the times must be changed to:

start: 14 February 19:00 (invisibly: UTC+3)
end:   14 February 23:00 (invisibly: UTC+3)

as opposed to remaining:

start: 14 February 19:00 (invisibly: UTC+2)
end:   14 February 23:00 (invisibly: UTC+2)

and thus on save becoming:

start: 14 February 20:00 UTC+3
end:   15 February 00:00 UTC+3

which is not what the user meant...

Change-Id: Ifce03e227b3f693e18e0588947bc32b7fc264369