priority specification

[Johannes Hirche]

> The possibility we propose is:
> p101) Save the priority as 10 values as RFC 2445 suggests:
>           0 = no priority selected
>           1-9 = priorities, with 1 being higest
> 
> p102) The clients have a choice of how to display those ten values
>           in principle. If a mapping is used towards low,  middle, high,
>           RFC2445 mapping must be used.
>  
>          We suggest to use the mapping of RFC 2445 for 
>  	  low, middle, high and "no selection" for all clients with
>           "no-selection" displayed and sorted as "middle" it not possible
>          otherwise.
> 
> p103) The clients shall preserve the saved value, even when 
>            using a different mapping for display, if the priority is not 
>            changed.
> 
> Joon, Till, Johannes:
> What do you think?

Fine with me, the only question which comes in mind is if (and if we do, how)
we should handle a migration of existing objects as we can't distinguish them.

Johannes