[mkcal] basic test for all day events
[mkcal] read event times with no timezone as clock time

Currently KDateTime::ClockTime is saved into Sqlite as that time in
current timezone. Reading such only works if timezone does not change.

Fix by assuming empty timezone as clock time, reading utc value and
changing spec to clock time. Thus clock time saved in one timezone
will be read as same time for every other timezone, too.