Rename the "none" damage class to "non-damaging".

Also decapitalize the classes' names, because they make sense that way
and they're not among the things the games always capitalize.

diff --git a/chrome/damage-classes/none.png b/chrome/damage-classes/non-damaging.png
similarity index 100%
rename from chrome/damage-classes/none.png
rename to chrome/damage-classes/non-damaging.png