Edit > Cut / Cut from layer: improve behavior when no selection is active

When using Cut from Layer without an active selection, the selected
layer should be deleted, per convention.

When using Cut from Image without an active selection, visible layers
should be wiped (e.g. made transparent).  They could be deleted, but
this causes trouble when considering the case of hidden layers, which
should not be affected by the operation; to circumvent that, visible
layers will simply be made transparent for now.