The main window's central widget has been changed to an MDI area. All
visualisation pipeline widgets are now displayed as subwindows in it.
This approach should help us to better utilise the space in the centre
of the main window.

git-svn-id: https://camplinux.in.tum.de/svn/campvis/branches/gui-redux@524 bb408c1c-ae56-11e1-83d9-df6b3e0c105e