Tk::DateEntry: set -weekstart => 1

DateEntry is used at two places, now all use Monday as first day of
the week.
activate_temp_blockings: check for <= and >=

This works better with "Sperrungen zeichnen für Datum", especially in
the case if a blocking starts at 00:00.
dump_future_as_perl: ignore undef records

That way the optimized temp_blockings file will not have useless
"undef" records at the beginning.
data update (minor)