I just checked the progress of the editor. Really fine work, Alex!

Will it be possible to sort the object-tree or to move objects up and down?

Yes this is a planned feature. I've added it to the Issue Tracker so I don't forget: http://quest.codeplex.com/workitem/618

A work around is to edit the XML to get them in the right order.

This feature was implemented in beta 4, so a workaround should no longer be necessary - I hope!