Allow WallPrototypes to use healing_per_tick

[MrSmoothieHuman]

As it says on the tin, I would personally like to have WallPrototypes be able to use healing_per_tick - it'd be a useful feature to have, as it means more possible types or sidegrades of walls are possible. I was specifically going to use it for a biological wall type I was developing for my overhaul mod,(which is the only reason im now making this request ).
Even if this isnt added, an explanation for what it means for an entity to be 'active' (as mentioned on the description of healing_per_tick) would be useful, because it seems to imply that any entity that isnt destroyed/unable to do something can use it? Maybe thats just how I read and interpreted it, but still :]

Thank you for reading!