fix board lookup for layer names with /

[gavin-ts]

"x.a" -> "y / b"

object ids can contain / so board lookup needs to account for this, passing the path as []string would be better.

If / is a valid character in layer names, wouldn't it also break this existing code?

// boardPath comes in the form of "x/layers/z/scenarios/a"
// or "layers/z/scenarios/a"
// or "x/z/a"
func (d *Diagram) GetBoard(boardPath string) *Diagram {
	path := strings.Split(boardPath, string(os.PathSeparator))
	if len(path) == 0 || len(boardPath) == 0 {
		return d
	}

If so, it seems like a bug.

Originally posted by @landmaj in #1725 (comment)

[landmaj]

I fixed this in #1725