[Folder View] Add another large icon size

The current set of icon sizes has three fairly small sizes, but only two reasonably large ones. Arguably only the largest size (128px) is truly "large" given the size of typical displays today. This makes the size distribution a bit unbalanced, and impairs the use case of wanting to have only a small number of very large icons on the desktop for display, presentation, or user assistance purposes.

Accordingly, this commit adds another large icon size to the top end of the options: enormous*2, which is 256px in practice. The context menu items are slightly renamed to accommodate the new size.

No config migration is needed since existing icon size steps are not changed.

cc @teams/vdg