iTip formatter: Hide the read-only calendars in the list
On receiving invitation the iTIP formatter shows a list of installed calendars (sources/destinations), where the invitation shall be stored. This list includes read-only calendars, like Birthdays and ics-subscriptions. Storing there the events is not possible, so there is no point to offer read-only calendars as choise.
If feasible, the same code from the CompEditor shall be used, so that the calendar color is also presented as part of the list.