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.
Calling wxDialog::EndModal() directly from inside the dialog is a
potential bug if the dialog is shown in the modeless or window modal
(quasi-modal). Use the internal button events where possible and
check for the appropriate mode before calling the correct end dialog
function.
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