The current behaviour is to prevent multi-user data from having its
transformation applied.
However in some particular cases it is possible to apply them:
* If all the users of the multi-user data are part of the selection.
* If not all the users are in the selection but the selection is made
single-user.
The active object is used as reference to set the transformation of the
other selected objects.
Note: For simplicity sake, this new behaviour is only available if all
the selection is using the same data.
This complements D14377.
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Review notes:
* For now this only works for scale. I want to have feedback/a pair
of eyes on the patch and functionality before implementing it for
rotation as well. (also I wouldn't mind help with those other cases :|)
* There may be a more elegant solution to handle single user for
multiple objects. But I haven't looked at it yet (poke Bastien ;)