Enable to re-use the same calendar for a number of frequencies in run-time settings

Current Situation

• Consider attached screenshot set_run-time_add-frequency,.png
  • The first entry makes use of a calendar and a restriction
  • Entries 2-4 cannot make use of the same calendar, i.e. they have to be specified without a calendar. The GUI does not allow to use the same calendar for a number of entries.

Desired Behavior

• It should be possible to use the same calendar for any number of frequencies.

Maintainer Notes

• This feature is dismissed for branch 1.x as there is no sufficient demand.
• Branch 2.x of JobScheduler includes more elaborate calendar features that address this issue.