Store and display status messages in a window similar to the Log window

It should be more visible imho, buried in Help menu for now, feel free to propose something about it.

![capture du 2013-07-06 11 53 02](https://f.cloud.github.com/assets/151042/756555/7eb4addc-e622-11e2-8aa8-0fac67886c70.png)

Note: this PR is very likely to conflict with https://github.com/musicbrainz/picard/pull/145 and https://github.com/musicbrainz/picard/pull/146