Fixed DAE export of groups including sub-groups and sub-parts

[ptenbrock]

Previously, the DAE file export for a selected group ended in an error message (variable vindex undefined). Now, all children of the group will be added as objects to be exported.

If objects in a group with a non-default Placement were exported, the placement in the resulting file was incorrect (the placement of the parent groups was ignored). This is now fixed.

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