Adds 'hidden' SOC value for Unlockables to absolutely hide unlockable from checklist

This re-adds the old functionality of 'nochecklist' that was previously commented out, but under a new name as to not conflict with its current use.

The 'hidden' value prevents given unlockables from ever appearing in the checklist. Useful for multiple menu unlockables that are meant to be unlocked as a group under one unlockable name to prevent clutter in the checklist menu, as well as allow for hidden permanent changes to a save file, much like how Red Volcano was hidden in Final Demo by clearing the game 3 times, but you were never told about it or that you've obtained the ability to go there in-game.

You will still get an unlock notification though, unless you also add NoCEcho to the unlockable.

Example Wad: hiddenUnlocksTest.wad