Grid settings: more options for isometric grids

[mgo]

It would be really nice if we were able to change all angles independently, and also choose between two and three axes. Grids then should come with a preset option, and be per document if possible.

[Greenlaw]

Yeah, I'd love to see more robust Grid options.

There should also be a global mode, not just per layer. For now, I have to create my own overlays to work around this limitation, but they don't support snapping, of course.