"Add Calendar" dialog should prevent the creation of duplicates (vs local calendar) when adding a remote calendar (with possibly multiple sub-calendars)

This is a continuation of #481 (closed). If I give a calendar a name when importing it, GNOME Calendar creates two new calendars: one empty local calendar with the given name, and one correctly-imported web calendar using a default name. It should only create the web calendar, and the web calendar should have the correct name.

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Additional notes: eventually, if going further than just preventing duplication (i.e.: ability to set a custom name and color for every online calendar being added at once), this would require a separate UI flow/dialog (and requires design), because creating a local calendar vs adding a remote one is completely different in terms of requirements and UX (under some circumstances it would need to prompt the user about which (sub)calendar to pick, for example); see #1037