[Menu, all builds, Dual-ISO] Rename submenu option "Every other frame" and dimm it in video mode

[WalterSchulz]

Expo screen -> Dual-ISO submenu.

Naming is poor. It doesn't affect movie mode but photo mode only and should be renamed "Every other pic" to avoid user confusion. Also caption extention telling it works in photo mode only may be useful.
Also: Should be dimmed in movie mode.