Add an option to delete all messages to the Prune Messages screen as per...

... #1589. Also spiced up the Prune warning a bit.

I made it go to removeall2 so not to bother the user any more with message boxes and dialogs and stuff, since they will receive a dialog already when hitting the button.

Signed-off-by: Yoshi2889 <rick.2889@gmail.com>