This can be useful to save the result of a cloth simulation as a
shape key without destroying the simulation, so it's possible to
e.g. re-run it to get other shapes, or simply use the new shape
key to start the simulation already in a draped state.
It also makes sense to allow applying as shape key even when the
mesh is shared, because the operation itself just adds a shape
key. To support this, split the apply operator into Apply and
Apply As Shapekey so that they can have different poll callbacks.
Differential Revision: https://developer.blender.org/D8173
01c8aa12a1