Snapping objects between the fine grid divisions #3283

Add a modifier that allows snapping to double the precision of the fine grid divisions so that users don't have to edit their preferences so often when switching between editing tasks.

I think this is basically a duplicate of #368, with the additional request for there to be a quick way to toggle which grid to snap to.

Alternatively, by giving scripts access to the Fine Grid preference, a custom action could be written that toggles between the fine grids.

Oh, I also like that behaviour! But separately.

Separately because it's not useful to lump it all in either a "room" grid and a "world" grid. There's another grid that needs jumping between in my use case: the "actual" fine grid. I just can't set my preferences to 4 divisions. It's so much effort and work to align things to such increments in every instance. But I do need the ability to do so when fine detail work arises.

Currently I set the fine grid at 2, so that all things are either on tiles or between them. But this prevents tasks such as building accurate polygons with the correct slopes, etc.