reimplement the javascript message box

This came up before the release and we postponed it till now.

There are two usecases that use the message box right now: errors from
the tagbox, and the login handling (temporarily disabled but will be needed again).

The component needs a message queue. In addition to the (html) message, an
optional parameter "queueCategory" can be specified when a status message is added.
The clear event would also accept this parameter to clean only a certain category
of messages.

This allows to handle different categories of messages and clear just the messages
of a given category.