[ Suggestion ] Grids should be per-artboard / only affect active artboard

I noticed the grid as it currently exists originates at the 0,0 point of a document, which is a problem if you use isometric grids on a document with multiple artboards.
Ideally grid origins should be able to be moved, for maximal freedom of creation, but in the meantime, they could at the very least only appear on the selected artboard's origin.

Start by locating the grid rendering path and the state that identifies the selected artboard. Compare the current document-origin behavior with the active-artboard requirement described here. Done means the grid appears only on the selected artboard, while movable grid origins remain a possible later enhancement.

Written by the indexing model from the issue text.