In the event editor dialog, don't show read-only remote calendars among the list of possible calendars to move / reassign an event to
This is the follow-up to @pksadiq's issue #57 (closed), where in in these commits:
- 9da68961 - manager: remove unused ::source-activated signal
- a76d40d2 - quick-add-popover: only show writable calendars
...the read-only calendars (here's an example remote calendar to set up in Evolution, for easy testing) were disabled from the "Add event" popover... however you need to also do that for the event editor's headerbar calendar picker widget thingy, to keep the UI clean.