Fix CalendarStyle::control as read-only
Fix CalendarStyle::control as read-only

This should never be set by the user, and marking it explicitly as
read-only makes it also appear read-only in the docs.

Change-Id: I9b66a62d2989446396e4c3ceb81d2ee8575d7a03
Reviewed-by: Mitch Curtis <a74594a5101269175fe1b140badc4c0b6f699d25@digia.com>

mizukusak/qtquickcontrols,mizukusak/qtquickcontrols,mizukusak/qtquickcontrols