Added a feat that allows darkvision toggling; which can be quickslotted or radial-toggled like any other state.
Adding this feat was a bit fiddly. For icons to appear in the radial menu, they must be added to the cls_feat_* lists - all of them. Due to this, the icon may appear multiple times in the class radial menu. This is something to be fixed clientside (NWNCX) in the future and cannot be helped at this time. While ugly, I don't think it's a big deal, as it only affects multiclassed characters and is hidden out of the way nearly all of the time.
As well, darkvision is now done solely through a visual effect - the engine effect itself (that the hardcoded feat grants) has been reverted to normal human vision. The upside of this is that darkvision got much slicker. It will no longer jump into normal vision as soon as a dynamic light source gets near enough; the player has control on when to enable it, as well as some scripted triggers that toggle it for you automatically (i,.e. when night falls). The downside is that possessed creatures may need additional tweaks to make it work well.
Please keep in mind though that this is a OOC-only feature for design aesthetics/personal preference. Most creatures do not actually have the ability to turn off their darkvision at will!