insert/move/resize/delete objects: undo/redo implemented

[Lutz Mueller <urc8 rz uni-karlsruhe de>]

Hi!

I just checked in complete support for undo/redo in combination with
object insertion, resizing, moving, and deletion.

Unfortunately, if you insert an object, click undo, and then redo, the
object will be smaller than before :-(

I'll try to figure out what is wrong here. I just wanted to let you know
about it.

Lutz

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