They don't define a KiCad string class, so the header file name was
somewhat misleading. But the fact that they didn't match definitely
made coding more difficult.
It's tempting to say that we don't need to exclude filename chars
from symbols, but we might regret that decision down the road. Better
to just escape them.
Fixes https://gitlab.com/kicad/code/kicad/issues/8694