Allow mods to override recipes

Fixes #6881

But there was a small problem with this:

The current code checks that each recipe has a unique ident and throws an error if recipes has the same ident (id of resulting item + `id_suffix`) as an existing one.

This is very helpful in case one adds a recipe without knowing that there is already a recipe for that item type.

Overriding an recipe without throwing can be done easily, but the game can not decide whether the overriding was intentionally (the mod wanted to change exactly this recipe) or not.

I added a flag `"override": false` to the json definition of recipes. If set to true it overrides any existing recipe definition.