svgedit: handle group dragging WIP

This is a start, but I don't get to the end with it now.

The actual setTranslate is moved to the DraggableObject itself,
which I think is good.

What I intended is to give this object a group property if it is
part of a group (in mousedown), but I don't see how to achieve that
(I thought I could compare to the draggable array, but didn't manage to.
Merge branch 'drag-groups' of https://github.com/PeterBjuhr/frescobaldi into svg-edit