Clone layers in Krita copy the projection of the source layer (and apply
some effects to it, like offset, masks and etc). But pass-through group
layers don't have any projection by definition, so clone layers cannot
copy them.

The only way to allow clone layers to copy pass-through groups is to
copy their "projection plane" and "projection leaf" instead of projection
device. But then there will be problems with masks, blending mode
and other funny problems we never thought about.

This patch just disables a clone layer that is connected to a pass-through
group and notifies the user about it by striking-through its name.

CCBUG:409949