event-editor-dialog: Distinguish hidden calendars

This MR will make the hidden calendars use an icon instead of the colored circle in case they're hidden, to indicate that they are not visible. This is done by introducing a new widget, GcalCalendarRow, which takes care of that.

Note that read-only calendars are displayed and can be selected, but that won't be a problem once !474 (merged) is merged.

Observations: The new widget needs some accessibility refinements, Orca won't announce that a calendar is hidden. I don't know how to do this, so I would really appreciate some documentation to learn how to implement it.

Along with !474 (merged), closes #950 (closed).

Related: !473 (merged).