[MlMt] Preview Pane should show last new item

[Michael Nietzold]

When:
- i select in 3Pane view some messages

what i see:
- when i select vom up to down then always the same first (by Date) 
message is in preview
- the same message is always reloaded on each new selected
- if i revert the order by Date then it is reverted: i always see the 
first message by Date

what i expect:
- always the new selected message should be in preview
- when i select from up to down then the last message
- when i select from down to up then the first message