Meeting invitation .ics event importer should not show read-only calendars among possible choices

If you open a .ics meeting invitation file from Nautilus (using "Open With…"), GNOME Calendar is able to handle it with its own import dialog:

However, this import dialog shows not only read+write calendars, but also read-only (ex: unauthenticated public webcal/HTTPS) calendars (such as the "On The Web" calendars seen in the above screenshot). That makes no sense, and can only end up with errors... and indeed, Calendar silently fails to add the event when you do that, the only thing that happens is this warning in the terminal:

GcalImportDialog-WARNING **: Error creating events: Cannot create calendar object: Not supported

This dropdown widget should hide read-only calendars, just like other event creation/modification places such as #957 (closed).