reenable sorting by sender in messagelist

This was disabled in faa66639
because it isn't possible to extract the name of the android
contact in the 'ORDER BY...' clause when querying the database.
Instead it simply sorts by the email address.

This may cause the same contact to appear multiple times in
the list, if they have multiple email addresses assigned.

But in most cases this is good enough and surely better than
not having the option to sort by sender at all.

Desktop mail clients such as Thunderbird also simply use the
sender email information when sorting the column.

This also adds a SenderComparator for usage in the MergeCursor.