Don't autohide messages but replace old ones.

This PR changes the behaviour of displaying portal messages from a closing overlay.

Motivation: Make sure that the most recent messages from the system are visible. When i close an overlay i'm interested in the overlay's feedback, not in old messages that are still visible from the initial page load.

Context: An user performs some actions with an overlay which is closed when the action finishes. The action's result might be displayed in a portal message. This message would be displayed inside the overlay, but the overlay is being closed, so the user can't actually see the message. Thus the message is moved to the main content column.

Old behaviour: The messages are shown below the last messages (if there is any) and then removed after 4 seconds.
New behaviour: The messages are shown forever and replace the previous messages.

@href what do you think? We've had the problem that users did not have enough time to read the new messages and then were confused about the old ones.