new attribute absolute_repeat for element period

The existing repeat attribute causes a start upon the start of the Job Scheduler and afterwards calculates the next start time from the end of the job (or order). Thus, the effective start times are dependant on the start time of the Job Scheduler and the time the job needs to complete. In other words, they are not predictable.
The new absolute_repeat attribute calculates the start times from the begin of the period. If no begin is specified, the default is 00:00. Start times generated with absolute_repeat are predictable.
If a job/order has not completed when the next start time is reached, that start will be skipped (in order to prevent the Job Scheduler from generating big task queues which will never be worked off.

Examples:
Run at 7:00, 7:15, 7:30, 7:45, 8:00 .... 18:15, 18:30, 18:45
<run_time>
<period begin="7:00" end="19:00" absolute_repeat="00:15">
</run_time>

Run at 7:00, 7:15, 7:30, 7:45, 8:00 .... 18:15, 18:30, 18:45, 19:00, 20:00, 21:00, 22:00, 23:00, 00:00, 01:00, 02:00, 03:00, 04:00, 05:00, 06:00
<run_time>
<period begin="7:00" end="19:00" absolute_repeat="00:15">
<period begin="19:00" end="07:00" absolute_repeat="01:00">
</run_time>