Improvements to occurrence calculation

[AntTBD]

Takes into account the start date of range ($_startDate) if not null to calculate occurrences only during the desired start and end date (without calculating from the 1st occurrence).

Description

When an agenda was created in 2017, for example, with occurrences every day, we arrive in 2024 and the time of display of the widget on the dashboard can take a lot of time and slow down the fluidity of the dashboard especially if one has several agenda widgets. Same problem with the duration of the display in the visualization for each agenda object (where you can choose planning/day/week/month/year.
OK, a first solution is to change the date of the 1st occurrence but sincerely in 1 year I would have forgotten the reason why my dashboard takes time to display and I will not often see if my calendar plugin is doing well...

Suggested changelog entry

EN: Calculation of occurrences only during the desired dates (instead of calculating from the 1st occurrence)
FR: Calcul des occurrences seulement pendant les dates souhaitées (au lieu de calculer depuis la 1ère occurrence)

Related issues/external references

Affichage widget lent sur le dashboard
Fixes #

Types of changes

• Bug fix (non-breaking change which fixes)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
  • This change is only breaking for integrators, not for external standards or end-users.
• Documentation improvement

PR checklist

• I have checked there is no other PR open for the same change.
• I have read the [La ligne directrice pour contribuer à ce projet / Contribution guidelines for this project).
• I grant the project the right to include and distribute the code under the GNU.
• I have added tests to cover my changes.
• I have verified that the code complies with the projects coding standards.
• [Required for new sniffs] I have added MD documentation for the sniff.