Introduce a world grid #3258

I have maps that align to a grid like this, but with a two-tile offset, because my maps have a 2-tile margin around them that isn't normally visible in-game. So, if possible, I'd love to see this feature implemented in such a way that I can snap to the world grid but then easily nudge the map over, e.g. with the keyboard as is currently possible. So, I guess the grid should only affect dragging, but not nudging.

Even cooler, but probably much harder to implement and only useful for edge cases like mine, would be some way such that dragging would snap to the world grid most of the time, but not when the move starts on the grid and is still within ~25% of the grid size away from the starting grid line, so that you get snapping behaviour for big moves, but can also make small adjustments afterwards, without changing snapping modes.

When this grid is enabled, it would be nice if maps newly added to the world snapped to it in addition to being placed next to the previously added map.

@eishiya Or maybe there would just be a modifier, like Ctrl, to temporarily switch on/off snapping to the world grid?

A modifier would certainly be easier to implement than context-sensitive snapping behaviour, I can deal with that. I guess when using the World Tool, if the world grid is enabled, it would snap to the world grid by default, and use the current snapping setting if the modifier is pressed, and if the world grid is not enabled, it dragging would work just like with objects with regard to snapping?

Oh, had a thought: when nudging, currently, holding Shift nudges it over 10 tiles or so, perhaps with the world grid enabled, shift+nudge could move it over [grid size] tiles instead? This would be great for rearranging a game (and unlike snapping, would preserve offsets).