CalendarStyle: document available styleData properties

Change-Id: I0eb2a3ef443fe0827cb6b5d1d4bfa44543025906
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
CalendarStyle: add styleData.index for dayOfWeekDelegate

The first/last day could be styled in a different way compared to the
days in the middle. Expose the index for convenience and consistency
with weekNumberDelegate.

Change-Id: Ia0cdf5d121f81c243345ea65a8ee355342b4f220
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
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 <mitch.curtis@digia.com>