P: Add Inverted Sky Mask to Landcape Mask Set

You can also create a preset based on an existing mask, but it would be nicer to create it at the point that you select the Landscape masking option.

A more general variation on that: If you hold down Opt/Alt while clicking a mask button or a checkbox (Landscape, People), it creates the inverse of the given mask component.

Of course you can always right-click the mask and do Invert afterward, but I agree with GoldingD that the common case of inverting skies, mountains, and other objects occurs often enough that it deserves a shortcut.