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.
Short net names are not unique; full names with paths must be used.
Added a grid cell renderer that does the escaping, to make it easier
to display net names in grid cells.
Once you unescape a net name, you can't go back to the escaped form
because you can't assume which `/` should be {slash} and which `/`
Because of this, we cannot use Unescape/Escape on the data model in
the netclass setup panel, and instead do the Unescape in the view.
Fixes https://gitlab.com/kicad/code/kicad/-/issues/5331
Jeff Young
Jon Evans