Make synced read-only events non-editable

[Frischid]

Checklist

• [X] I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• [X] I made sure that there are no existing discussions - open or closed - to which I could contribute my information.
• [X] I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
• [X] I have taken the time to fill in all the required details. I understand that the bug report will be dismissed otherwise.
• [X] This issue contains only one feature request.
• [X] I have read and understood the contribution guidelines.
• [ ] I optionally donated to support the Fossify mission.

Feature description

If a read-only calendar is synced with davx5, fossify calendar allows to edit events even though the changes can not be synced since it is read-only.

Why do you want this feature?

Editing a read-only event makes it inconsistent since there is no indication that the changes are not synced.

Additional information

Other calendar apps, e.g. aCalendar, show the read-only davx5 calendar properly as write protected read-only and do not allow editing. Thus, it seems that the read-only property is somehow available.