Filter out read-only calendars

Hari Ranarequested to merge
TheEvilSkeleton/gnome-calendar:filter-out-ro into main

This MR introduces a getter method that retrieves read-write calendars. The event editor and importer dialogs now use that method to only list read-write calendars.

Closes #957 (closed), #1134 (closed)

Edited by Hari Rana

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