Rework stealth adjective scale to steps of 50

(And one of 100 at the end.)

I'm not yet fully convinced linearity is better than the old progression.
This does however look not too intrusive, and might enable presentation in form
of a meaningful stealth bar at some point.
Rings/artefacts of stealth (new scale) always affect the stealth description
until 400 stealth. Some other sources could be tweaked a bit to fit in as well.

Then of course there are smaller factors like racial stealth (as for MR).
Together with certain multipliers like from being in water, backlit, encumbered
and so on, changing them to fit into one dead-set scale is neither desired nor
in scope of these changes.