Also simplifies the dialog by removing the user grid (now that you
can add as many user grids as you want).
This does mean that you can no longer have an asymmetric grid, but
it gets too complex if we allow everything for everyone.
Add a reset button that will only show up if the user
edits the JSON configuration for grids. This is a temporary
measure until we have a real grid editor GUI in V7.
Fixes https://gitlab.com/kicad/code/kicad/-/issues/7595