For objects with shared data, it makes sense to show the mode icon for every
object sharing the same data if one of them is in edit mode.
This also disables the "extend" functionality in this case, because
being in edit mode for multiple objects with the same data isn't supported.
It might be nice to distinguish the "shared data edit modes" from the regular
buttons a bit, but we don't have great icons for that.
Depends on D9272